This ultra-creamy Brussels Sprout and Potato Gratin Recipe with Crispy Fried Onions is the perfect holiday side dish for even the pickiest of veggie eaters. Made with real ingredients (no canned soup), Brussels sprouts, potatoes, and gooey melted gruyere and cheddar cheese, this beautiful gratin is guaranteed to be a new holiday favorite!

Brussels Sprout and Potato Gratin Recipe

Thanksgiving has come and gone. Let’s talk about Christmahunakkah!

Don’t worry, you guys! I’ve got us covered. For starters, check out this Persimmon Pumpkin Pudding Cake. OMG, the easiest cake ever and so super delicious!

But first, let’s chat about GRATIN! Brussels Sprouts and Potato Gratin (to be specific) which just so happens to be perfect for both Thanksgiving and Christmas. Rather than make a side for Brussels sprouts and a side for potatoes, let’s just combine the two into one luxurious side and make everyone happy.

It workes beautifully, you guys. Trust me.

Of course, being the holidays and all, I highly recommend that you include some kind of sweet potato dish (because they’re just so good!) and maybe some mashed potatoes (because why not?) But really, this creamy, cheesy and super veggie-packed gratin recipe will not disappoint. And those fried onions on top? Don’t skip those. They’re like the green bean casserole icing on top.

Love Brussels sprouts but prefer something a little lighter? Try my Roasted Brussels Sprouts or Simple Shredded Brussels Sprout Salad (personal fav).

How to make Brussels Sprout and Potato Gratin

Start by preheating your oven and bringing a large pot of salted water to a boil. These things take time (especially the water boiling part) so it’s best to get them out of the way. Set your oven to 375 degrees F and bring a large stockpot of salted water to a boil.

Halve your Brussels Sprouts and slice your potatoes into 1/4″ rounds. Once your pot of water is boiling, blanch the Brussels sprouts and potatoes. Simply add the Brussels Sprouts and potatoes to the pot and boil for 3-4 minutes (do not overcook or the potatoes will not maintain their shape and the Brussels sprouts will lose their bright green color). Drain and immediately transfer to a large pot filled with ice water to stop them from cooking any further. When the sprouts are cooled, drain, and set aside.

Meanwhile, start the sauce. Melt butter in a large skillet over medium-high heat and add the minced shallots. Sauté for 3-4 minutes, then add your freshly chopped thyme, rosemary, and garlic and continue to cook for 1 minute more. Reduce heat to medium and sprinkle the flour on top of the vegetables. Stir to combine, taking care not to burn the flour. Add the marsala wine and whisk well to combine. Cook for one more minute.

Finish your sauce. Add the milk and half-and-half to the skillet and whisk well until the sauce is smooth and thickened. Remove from heat and mix in half of the shredded cheese until completely smooth and melted. Season with salt and pepper. Add the Brussels Sprouts and potatoes to the cheese sauce and mix well to combine.

Transfer to a gratin dish and bake. Transfer to a greased 8×10 baking dish and cover loosely with foil. Bake for 30 minutes. Sprinkle with fried onions and the remaining shredded cheese and return to the oven. Set oven to broil and cook for approximately 2-3 minutes, or until the cheese is golden and melted.

Frequently asked questions

I feel like casseroles tend to come with loads of questions. I’m going to do my best to answer what I can ahead of time, but please feel free to leave me a comment if you have any questions of your own.

What is Au Gratin?

The simple definition for au gratin is that it is something, usually a casserole, that has been sprinkled with breadcrumbs or grated cheese, often both, and broiled until browned and melted. These are au gratin potatoes and Brussels sprouts, or simply, a Brussels sprouts and potato gratin (except that here I sprinkled with fried onions rather than breadcrumbs).

In other words, a gratin is a casserole. But gratin sounds fancier, so I’m going to use it.

What type of cheese should I use?

This really depends on personal taste and preference. With that said, the most popular cheeses for gratin recipes include nice aged cheddar and gruyere. Each shreds beautiful and melt so silky smooth. Sometimes I’ll add a small handful of parmesan for its sharp taste and added flavor.

Do I have to use an au gratin pan?

No. Au gratin pans, usually shallow and oval in shape are typically chosen for au gratin recipes because they expose ingredients to heat more directly. That said, you do not have to use an au gratin pan when making this recipe or any other grain recipe.

What should I serve with Brussels Sprout and Potato Gratin?

Feel free to serve this amazing and creamy side dish with a variety of recipes,

Can I make this ahead of time?

Yes! It’s super easy to make au gratin Brussel sprouts in advance. Assemble the dish according to the recipe instructions, but do not add the fried onion and cheese topping. Cover your dish with foil or plastic wrap and transfer to the refrigerator. When ready to cook, allow a few minutes for your casserole to come to room temperature then bake as instructed. Keep in mind that you may need to add an additional 5-10 minutes to the cooking time.

Can I freeze this Brussels Sprout and Potato Grain Recipe?

Yes! Assemble your casserole according to recipe instructions. After it’s been baked, remove from the oven and allow it to cool completely before covering tightly with a layer of plastic wrap, followed by a layer of aluminum foil. Transfer to the freezer for up to 2-3 months.

More holiday recipes,

If you try making this Brussels Sprout and Potato Gratin Recipe, please leave me a comment and let me know! I always love to read your thoughts and feedback!

Brussels Sprout and Potato Gratin Recipe