Cheesy Sweet Potato Gratin

The first time I made this gratin, I was trying to impress my new in-laws at Thanksgiving. I spent forty minutes standing over the stove, watching onions transform from sharp and pale into something golden and unrecognizable, wondering if all that fuss was worth it. When my father-in-law took his first bite and literally closed his eyes, I knew I had stumbled onto something special. Now it is the one dish people actually ask for by name.

Last winter, my neighbor Sarah came over during a snowstorm with nothing but a bottle of wine and an appetite. We stood in my kitchen while the gratin bubbled away, the smell of melting Gruyère and onions filling every corner of the house. By the time we finally sat down to eat, the snow had piled up against the windows and neither of us cared one bit.

Ingredients

• 2 lbs sweet potatoes, peeled and thinly sliced: A mandoline creates perfectly even slices, but a sharp knife and steady hand work just fine. Choose potatoes that feel heavy and firm without any soft spots.
• 2 large yellow onions, thinly sliced: Take your time here. Even slices mean even caramelization, which is the secret to that deep, rich flavor throughout.
• 2 cups shredded Gruyère cheese: Gruyère melts beautifully and adds that nutty, sophisticated flavor that makes this feel special. Buy a block and shred it yourself for the best texture.
• 1 cup shredded sharp cheddar cheese: The sharpness cuts through all that richness and adds a familiar, comforting note that everyone recognizes immediately.
• 1 1/2 cups heavy cream: Do not be tempted to use half-and-half here. The cream creates that luxurious, velvety texture that defines a proper gratin.
• 1/2 cup whole milk: The milk lightens the cream just enough so the dish does not feel overwhelmingly heavy.
• 3 tbsp unsalted butter: Use two tablespoons for the onions and one for the cream mixture. Every bit contributes to that golden, buttery finish.
• 1/2 tsp dried thyme: Fresh thyme is wonderful if you have it, but dried works perfectly here and distributes evenly throughout the layers.
• 1/4 tsp ground nutmeg: Just enough to add warmth without making the dish taste like dessert. A little goes a long way.

Instructions

Get your oven ready:

Preheat to 375°F and butter a 9x13 baking dish thoroughly. This step matters because those edges will get golden and crispy, and you want every bite to be worth savoring.

Transform the onions:

Melt two tablespoons butter in a large skillet over medium heat, add the onions with a pinch of salt, and cook them slowly for 20 to 25 minutes. Stir often and watch as they turn from white to golden to deep brown. Add the garlic for just one minute at the end.

Warm the cream mixture:

In a small saucepan, combine the cream, milk, thyme, nutmeg, pepper, remaining butter, and a half teaspoon of salt. Heat until steaming but not boiling. Whisk in the flour now if you are using it.

Build the first layer:

Arrange half the sweet potato slices in the baking dish. Sprinkle them lightly with salt, then spread half the caramelized onions on top, followed by half of each cheese.

Repeat and pour:

Layer the remaining potatoes, onions, and cheeses, then pour that warm cream mixture evenly over everything. Take a moment to admire how beautiful it looks before it goes into the oven.

Bake covered:

Cover the dish tightly with foil and bake for 40 minutes. The foil traps all that steam and helps the potatoes cook through evenly without drying out.

Uncover and golden:

Remove the foil and bake for another 20 minutes until the top is bubbling and golden brown. If the cheese is not quite brown enough, give it a quick pass under the broiler.

Rest before serving:

Let the gratin sit for 10 minutes before serving. This feels impossible when everyone is hovering and hungry, but it helps the layers set so you can scoop out perfect portions.

Save

My daughter requested this gratin for her birthday dinner instead of cake. We ate it straight from the baking dish while watching a movie, and she declared it better than any restaurant meal she had ever had. Some traditions are worth starting.

Make It Ahead

Assemble the entire gratin up to two days in advance, cover tightly, and refrigerate. Add ten minutes to the covered baking time if it is cold from the fridge. The flavors actually meld and improve overnight.

Choosing Your Cheese

Gruyère can be pricey, but this is one place where the investment shows. If you need to substitute, try Fontina for excellent melting or Emmental for a milder flavor. Avoid pre-shredded cheese which contains anti-caking agents that prevent smooth melting.

Serving Suggestions

This gratin pairs beautifully with roasted meats or stands alone as a vegetarian main course. A green salad with sharp vinaigrette cuts through the richness. Add these final touches for a memorable meal:

• Top with fresh chives or parsley right before serving for a pop of color
• A crisp white wine like Sauvignon Blanc or a light red like Pinot Noir complements the creamy texture
• Serve with something acidic like roasted tomatoes or a lemon-dressed salad

Save

There is something deeply satisfying about a dish that transforms simple ingredients into something extraordinary. This gratin has earned its permanent place in my regular rotation.

Recipe FAQs

→ How do you caramelize onions properly?

Cook sliced onions slowly over medium heat with butter and a pinch of salt, stirring occasionally until they turn deep golden brown and develop natural sweetness, usually 20-25 minutes.

→ Can this dish be made gluten-free?

Yes, by using gluten-free flour for thickening and verifying cheese labels are free from additives, this gratin can suit gluten-sensitive diets.

→ What cheeses work best for this gratin?

Gruyère and sharp cheddar provide a rich, melty texture and balanced flavor that complement the sweetness of the potatoes and onions.

→ How should the gratin be baked?

Bake covered for 40 minutes to cook through, then uncovered for 20 minutes to develop a golden, slightly crisp top layer.

→ What are good serving suggestions?

Serve it warm as a hearty side dish or main, paired well with crisp white wines like Sauvignon Blanc or light reds such as Pinot Noir.

→ Can this dish be prepared ahead of time?

Yes, it can be assembled in advance and gently reheated before serving without losing its creamy texture.