Potatoes Au Gratin with Caramelized Onions

This gorgeous casserole dish of Potatoes Au Gratin with Caramelized Onions is a little more sophisticated, and tastier, than it’s scalloped potato cousin but just as easy to prepare and equally family friendly. Soul-satisfying, comforting, and nostalgic, this is hands down the best gratin dish you’ll ever taste.

This recipe takes the classic scalloped potatoes and makes them scandalously richer, fancier, and one hundred times tastier. A cheesy, onion deliciousness that everyone goes crazy for. The only real extra step is caramelizing a small heap of onions which takes a little time but so, so worth it, you’ll see.

I have just one rule with this dish: I serve it only once a year. Okay, maybe twice, when the begging gets really loud.

Potatoes Au Gratin With Caramelized Onions

For as long as I can remember, this potato au gratin dish with caramelized onions has made an appearance every single year at our holiday table. There would be a full-on revolt in our household if it didn’t. The combination of thinly sliced potatoes, silky caramelized onions, threads of melted gruyere cheese throughout, and bubbling heavy cream creates an absolutely outrageous dish that everyone loves. One bite in, your eyes will roll back and your knees will buckle. No joke.

The recipe is one I adapted years ago from The Barefoot Contessa Cookbook – so you know it’s decadent, you know it’s foolproof, and you know it’s beyond delicious. The major change in this recipe is that I omit the fennel. Though I absolutely adore fennel, others prefer it without. This omission turned out to be a good thing because it allowed me to take the onions to the caramelized side of the spectrum.

In my opinion, this deep caramelization is what makes this potatoes au gratin so deluxe and so incredibly good. They are scrumptious served warm, and just as good reheated the next day. So tasty, I have no doubt this will become your favourite new holiday dish.

Difference Between Potatoes Au Gratin And Scalloped Potatoes

This gorgeous potatoes au gratin dish uses few ingredients and is much simpler to make than you imagine.

• Potatoes – This recipe calls for 3 pounds of russet potatoes which is roughly 5 medium-large. If possible, buy them similar in size and shape so the potatoes will bake more evenly. See below for more on potatoes.
• Heavy cream – We use 2 1/4 cups heavy cream. I have tried replacing some of the cream with milk but this occasionally causes the sauce to split. The amount of cream is scary, I know. Just adopt my once-a-year-rule and enjoy it.
• Onion – You will need a large Spanish onion, or two medium yellow. It will seem like a lot of onion but they cook down drastically once caramelized. If you don’t have too much time to caramelize the onions, just sauté them for 8 to 10 minutes. They will still be delicious.
• Gruyère cheese – Gruyère is the best cheese for this potatoes au gratin dish. It is notoriously melty, has a beautiful creamy texture, and the flavour is rich and slightly nutty but not overwhelming. It is sold in most well-stocked grocery stores (Costco’s Gruyere is excellent and has the best price) so you shouldn’t have a hard time finding it. If you need a substitute, try Emmental or Jarlsberg.
• Salt & Pepper – Potatoes and cream can be bland so season generously.
• Fresh Thyme – Optional for garnish.

What Are The Best Potatoes For Potatoes Au Gratin?

The key to success when making potatoes au gratin is choosing the right potatoes. Starchy potatoes such as Yukon Gold or Russet (also known as baking potatoes) are best for potatoes au gratin. The starch helps thicken the sauce and make it nice and silky. They also aren’t too firm so they will bake up nice and tender.

Though I love Yukon Gold potatoes, for my potato gratin I prefer to use Russet. Yukon Gold’s are a little firmer so they won’t fall apart. However, I like that Russets cook a bit faster and will be more tender. Nothing worse than under-cooked potatoes even after a long baking time.

My best advice, use what you can easily find and become familiar with how they bake.

How To Make Potatoes Au Gratin With Caramelized Onions

Even this fancied-up version of scalloped potatoes is very straightforward and easy to throw together. Here is what you do:

• In a medium/large skil­let, heat butter and oil over medium-low heat. Cook onion slices, stir­ring often, until light brown, about 30 to 35 min­utes.

   

• When onions are almost done  caramelizing, heat the cream and salt in a medium saucepan over medium heat just until warm, about 4 to 5 minutes. Slice pota­toes into 1/8-inch thick rounds (a man­do­line makes this job easy).

   

• Layer half of the potatoes slices in the prepared casserole dish making sure to separate any slices that are sticking together. Pour half of the warm cream over the potatoes; top with all of the caramelized onion and a little more than half of the cheese. Layer the remaining potato slices on top and drizzle all the remaining cream evenly over the potatoes. Push down gently on the potatoes with your hands to ensure they are well covered in cream. Sprin­kle with remain­ing cheese.

   

   

• Cover tightly with foil and bake for 45 minutes. Remove foil and bake until potatoes are tender and golden brown, about 30 minutes longer.
• Remove from oven and let rest for a few minutes before serving.

• Soak the sliced potatoes if not using immediately. I try and wait until the onions are caramelized before peeling and slicing the potatoes. This way they don’t turn brown and I can avoid having to soak them. If you aren’t ready to assemble the casserole but you have already sliced the potatoes, soak them in a large bowl with cold water. When ready to use, drain them and dry them as best you can with a kitchen towel.
• Slice the potatoes evenly and thinly. You don’t want to slice your potatoes so thin that they will turn to mush. However, they need to be sliced thin or they will not get tender even after a long bake time. I aim for a uniform 1/8-inch thickness. A mandolin makes this job really easy and fast. I have, and love, this one.
• Warm the cream. Gently heat the cream on the stovetop before pouring over the potatoes. The warm cream will start to help the potatoes soften and release their starch.
• Bake covered for half the bake time. Covering the dish with foil creates steam which helps cook the potatoes faster and prevents them from drying out. This also prevents the potatoes from browning too quickly. The foil is then removed during the last half hour so the casserole gets nice and golden on top.
• Cool slightly before serving. Allowing the casserole to cool slightly before serving will help the potatoes absorb more sauce and also make it easier to slice and serve.

Make Ahead

Potatoes au gratin can easily be made ahead. The trick is to be sure the potatoes slices are completely immersed in sauce, and covered tightly so they  don’t turn brown.

Simply assemble the entire dish, cover tightly with plastic wrap, and refrigerate for up to 24 hours. Bake as instructed adding about 10 minutes to the cooking time.

Alternatively, bake the dish a day or two ahead, then reheat in a 350º, covered in foil, until warmed through. Reheating the potatoes au gratin makes them a little less creamy but they will still be delicious.

To Store

Leftovers keep well, covered tightly, for up to 5 days in the refrigerator. When ready to use, reheat them in the microwave for 1-2 minutes until heated through or warm them, covered in foil, in a 350°F oven until heated through, about 15-30 minutes.

To Serve

Let’s just say if there is a roasted turkey in the oven, these potatoes au gratin with caramelized onions are making an appearance. Here’s our favourite mains to serve with this potato au gratin:

• Roast Turkey Breast with Braised Red Cabbage
• Spatchcock Turkey With Dry Brine
• Roasted Chicken and Root Vegetable

More Holiday Dishes To Try

• Red Wine Cranberry Sauce
• Balsamic Roasted Shallots
• Savoury Sweet Potato Casserole
• Sweet Potato Gratin with Sage

If  you give this Potatoes Au Gratin with Caramelized Onions recipe a try, please feel free to leave a comment and/or a star rating below. We appreciate and welcome all your feedback. Thank you!