Potato and Cauliflower Gratin with Truffle Oil

by angelika on September 24, 2010

We had a cold night, it felt like fall was indeed sweeping in, right on cue. Never mind it will be back in the 80’s all weekend, for a minute we had a glimpse. Fall is my favorite time of year. To my mind everything good happens in fall. We start with my birthday, then we continue on to Halloween, we come around the corner to the cooking olympics, otherwise known as Thanksgiving, which also just so happens to also be little man’s birthday.  After Thanksgiving we have the Christmas season, which really, all happens in fall save the actual Christmas day. We have a big Solstice Party every year to cap off a glorious Autumn. It doesn’t hurt that fall is resplendent with hues of gold and copper, rust and myriad shades of green. These are very good colors for me. Just saying.

So on this lone cold night I felt moved to make one of my favorite comfort dishes. A gratin. We eat a lot of gratins in our home. A lot. Seriously though, is there a vegetable on earth that doesn’t taste better smothered in a cream sauce? You will probably eventually see at least a dozen incarnations of gratin on this blog. For now, we will make a Potato and Cauliflower gratin. mmmm.

Potato and Cauliflower Gratin with Truffle Oil
4-5 potatoes, I used russet  (yukon gold are better)
1 head of cauliflower (mine was big and I had lots of cauliflower left over)
1 TBS salt for water
1 yellow onion or 3 shallots, finely chopped 
2TBS ghee or butter
pinch salt
3 cloves garlic, minced
1/4 cup white wine
3 TBS butter
3 TBS flour
1 pint heavy creme or half and half
7-8 oz Gruyere, shredded  
1/2 tsp thyme
1/8 tsp nutmeg
2 TBS truffle oil (optional)
1/2 tsp salt
1/4 tsp pepper
To begin have a pot of water with 1TBS salt waiting, and preheat the oven to 375º. We then need to slice the potatoes very thin. I like to use a Mandoline. This is another kitchen gadget that was a gift that I thought I would never use. Boy was I wrong. I use it all the time. If you don’t have one you can cut the potatoes by hand, but try to get them as thin as possible. The thicker they are the longer the gratin has to cook. 
I set my Mandoline to 1/8″ thickness

You want to add all your sliced potatoes to the pot of water, but don’t turn it on until you have it full. Once you have added all your potatoes turn the heat on to high and set a timer for 15 min. This is the best method I have found for getting the right amount of pre-cooking. You want them just starting to cook, if you cook them too long they fall apart when you try to assemble the gratin. If that happens to you, don’t fret. It will still taste great, and it has happened to the best of us. After the 15 minutes drain the potatoes and leave them in the sink to cool a bit. Do not rinse.

Next you prepare the cauliflower. You want to remove the core, and then slice the head in half. Then try to get long fan shapes by cutting in 1/2 inch thickness. This insures even cooking, as the fleurettes can be too big. It wouldn’t hurt to precook these for a few minutes either, but I did not. If you want you can add them for the last 5 minutes of the potatoes, but separating the two can be a bit hard and I wanted nice pictures. 
Now while the potatoes are cooling we make the sauce. This is a basic French white sauce which is a good sauce to master. You can do so many things with it, and it is the base of many other dishes. Tonight the sauce happens in two stages. In one pot the roux and white sauce base, and in the same pot I had the potatoes in I make the garlic and onion addition. The reason I use two pans is simple. I need to be able to whisk the cream sauce when I incorporate both the flour and the cheese. The onion and garlic bits make this difficult. Much easier this way! 
Chop your onion to a fine dice, and add it to 2 TBS ghee or butter on medium heat with a pinch of salt.  I love to use ghee as it is more forgiving than butter. It has a higher burn temperature and it lasts longer. You can use either, or even olive oil. I only had onion, but I have use shallot before in this dish and the taste is truly divine. So if you have shallots, or you are making a special trip to the store to make this recipe go for the shallots! You want to soften and lightly brown the onions/shallots. This will take about 7 minutes. When the onions are done add the 3 cloves of garlic which should be finely minced. Saute for a minute and then deglaze with the white wine. A dry white wine is best, but I used Chardonnay since I had it open. Saute off the alcohol and scrape the bottom of the pan with a wooden spoon to loosed all the bits. Turn off heat and set aside. 
all finished
To make the white sauce we start with 3 TBS of butter. We want the milk solids in this so don’t use ghee. (which is clarified and the milk solids are removed) Heat on medium low until melted, then to this add 3 TBS flour. (I used whole wheat pastry) Whisk constantly and let the flour cook for about 2 minutes. 
roux with flour cooked, it darkens a touch
To this add the cream.  Now I have used heavy cream, half and half and even plain 2% milk. They all work, but vary a lot in both richness and texture. Clearly the cream is the best, but if you want to lighten it up a bit feel free. Now you continue to heat on medium low whisking until the cream and roux thicken. This will happen just as the mixture comes to boil. 
perfect thickness

Once the mixture has thickened turn off the heat,  add in your thyme and nutmeg, and then whisk in the cheese. Don’t add the salt until after you have added the cheese, as gruyere varies in saltiness. Add your salt and pepper and taste, and then your onion mixture. At this point, if you have truffle oil, you add it.

finished sauce 
Now you absolutely don’t have to add the truffle oil, but my husband and I both happen to be absolutely, positively, besotted by both truffle oil and truffle butter, and for that matter, the entire truffle family. A truffle is mushroom that grows underground and is considered a delicacy in French, Spanish and Italian cooking.  In this regard, I submit,  there is perhaps no better combination than truffle, gruyere and creme. I prefer the white truffle which is native to Italy, but if all you have is black truffle I won’t turn you away. In this recipe I used white truffle oil. Frankly I felt there was not enough truffle to my taste and next time I may add an additional TBS, but that’s just me. 
Finally you assemble the gratin. You can use any number of dishes. I have a Le Creuset gratin pan that I often use, but I felt it was too small for this dish. I actually love to use this funny rounded ceramic baker I have. Once upon a time it was the bottom to an Emile Henry rice cooker, which had a conical top promising to cook my grains perfectly every time. When my son was young he broke the top. It was a sad sad day. It took a year or so before I was over the injury enough to realize that the bottom still had use; I love it for deep gratins, and it is also wonderful for baking rounded bread loaves! If you are using a shallow dish you may want to test for doneness earlier than mine. You do this by inserting a knife and seeing if it goes in and out easily. 
ready to assemble

Butter the inside of your vessel. Then put one ladle of sauce in the bottom and spread it around. Then layer the potatoes about two to three deep. Salt and pepper each layer. It will seem like too much salt, but the potatoes really need it. 

Then you add a layer of sauce, and a layer of cauliflower. There is no real rule about how much cauliflower you add, if you want a hint make it a small layer. If you love cauliflower make it a thick layer. If you don’t like cauliflower, make it all potato! 
To the cauliflower add another layer of sauce and spread it evenly. Layer your potatoes on top and cover in sauce. 

ready for the oven

Cover with a piece of tin foil coated on the inside with butter. This prevents the cheese from sticking to the foil. Bake at 375º for 30 minutes. Uncover and bake for an additional 10-15 minutes or until the gratin tests done and the top is golden and bubbly. You definitely want to put your vessel on a sheet pan of some sort. This is likely to spill over a tad. 

Let the gratin rest for at least 15 minutes if you have a deep dish like mine. You need the dish to cool in order for it to set up and cut clean. This will usually take 20- 30 minutes. Confession- I could not wait. I just couldn’t, I was too excited. What happens when you can’t wait is that you have a piping hot dish, for which you have to blow on each bite, both for yourself and your children, but it tastes divine and there is a satisfaction that can’t be beat. What you lose is the prettiness of the clean cut and the deep gratin looking like a gorgeous piece of pie. Instead you have a gooey yumminess spilling on your plate. 
If you are serving this for a dinner party or a holiday, wait. It is worth it. The pieces look super pretty. If you just want to eat, then go right ahead. If I can’t wait I surely don’t expect you to! 
A votre santé! 
PS- I feel a little guilty that all my posts have been rich and decadent. What kind of vegetarian health nut am I? Well, to make myself feel better, I will post my favorite salads this weekend. As when you eat something rich, decadent and divine, best to pair it with a nice healthy salad so you can sleep at night. 

{ 5 comments… read them below or add one }

Grandma Wendy September 24, 2010 at 9:41 pm

Soooooo . . . is your favorite salad your Fattoush? :-)


Angelika Mitchell September 24, 2010 at 11:44 pm

Actually Wendy, I am sorry to tell you my favorite salad is an arugula salad. Yikes! For you I will also post the recipe to the Fattoush, since you clearly won't be rushing out to prepare arugula. 😉


Grammy in CA October 15, 2010 at 10:49 pm

Oh my… I just discovered your blog. You have re-ignited my desire to cook in my beautiful kitchen my husband recently remodeled for me! I can't remember who recommended your site, but I'm SO GLAD that I clicked on the link that was posted!

Thank you, Thank You, THANK YOU!!


Angelika Mitchell October 16, 2010 at 3:10 am

Thank you so much Grammy in CA. You made my night! Enjoy your new kitchen. What a sweet husband you have. :)


Shelby October 28, 2010 at 2:13 am

oh my! This was sooo delicious! Trell could not get enough. He went back for seconds. Then went and ate more from the dish. Then I caught him eating it again as he was putting it away!


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