Cheesy Sweet Potato Gratin

The first time I made this gratin, my kitchen filled with the most incredible aroma of sweet, caramelizing onions and nutmeg that had my whole family wandering in to see what was happening. I had actually intended to make a regular potato gratin, but discovered I'd bought sweet potatoes instead and decided to embrace the mistake.

I served this at a fall dinner party last year, and my friend Sarah who claims to hate sweet potatoes went back for thirds. Watching people's faces when they take that first bite of creamy, cheesy goodness is honestly half the fun of making it.

Ingredients

• 2 lbs sweet potatoes: I've learned that slicing these about 1/8 inch thick gives you that perfect tender texture without falling apart
• 2 large yellow onions: Take your time caramelizing them—this is where all that deep, savory flavor comes from
• 2 cups shredded Gruyère cheese: This cheese melts into the most luxurious, creamy sauce you've ever tasted
• 1 cup shredded sharp cheddar: Adds that extra punch of flavor that keeps things from being too mellow
• 1 1/2 cups heavy cream: Don't skip this—the rich sauce is what transforms it from good to extraordinary
• 1/2 cup whole milk: Lightens things just enough so the cream doesn't become overwhelming
• 3 tbsp unsalted butter: Used both for caramelizing the onions and enriching the cream mixture
• 1/2 tsp dried thyme: Such a small amount, but it adds this lovely earthy note that ties everything together
• 1/4 tsp ground nutmeg: The secret ingredient that makes this taste like something from a fancy French bistro
• 2 cloves garlic: Add these at the very end with the onions so they don't burn

Instructions

Get everything ready:

Preheat your oven to 375°F and butter a 9x13-inch baking dish generously

Caramelize the onions:

Melt 2 tablespoons butter in a large skillet over medium heat, add the sliced onions with a pinch of salt, and let them cook down slowly for 20-25 minutes until they're deep golden brown

Add the garlic:

Stir in the minced garlic and cook for just one minute more, then remove the pan from heat

Make the creamy sauce:

In a small saucepan, warm the cream, milk, thyme, nutmeg, pepper, remaining butter, and salt over medium-low heat until steaming

Start layering:

Arrange half the sweet potato slices in the bottom of your prepared dish, season with a little salt, then spread half the caramelized onions on top

Add the cheese:

Sprinkle half of both cheeses over the onions, then repeat everything with the remaining potatoes, onions, and cheese

Pour and bake covered:

Pour that warm cream mixture evenly over everything, cover tightly with foil, and bake for 40 minutes

Get that golden top:

Remove the foil and bake another 20 minutes until the top is bubbling and golden brown

The waiting game:

Let it rest for 10 minutes before serving—this helps everything set so your slices hold together beautifully

Save

This gratin has become my go-to for bringing to friends' houses during the holidays. Something about the combination of sweet and savory just makes people feel at home.

Make It Your Own

I've discovered that adding a layer of sautéed mushrooms between the potato layers creates this amazing earthy depth. A pinch of smoked paprika in the cream mixture adds a subtle warmth that's particularly nice on cold winter evenings.

The Make-Ahead Secret

You can assemble the entire gratin a day ahead and keep it in the refrigerator—just add about 10 minutes to the covered baking time if you're baking it cold. I actually think the flavors meld together even better this way.

Serving Ideas

This works beautifully as a main course with a simple green salad dressed with something acidic to cut through all that richness. It pairs wonderfully with roasted chicken or can stand alone as a vegetarian centerpiece.

• Try topping with fresh chives right before serving for a pop of color
• A crisp white wine like Sauvignon Blanc cuts through the creaminess perfectly
• Leftovers reheat surprisingly well in the oven at 350°F for about 15 minutes

Save

There's something deeply satisfying about serving a dish that looks this impressive and tastes this good. Hope it becomes a favorite in your kitchen too.

Recipe FAQs

→ How do you caramelize the onions properly?

Cook thinly sliced onions slowly over medium heat with butter and a pinch of salt, stirring often until deeply golden and sweet, usually 20-25 minutes.

→ Can I prepare this gratin ahead of time?

Yes, assemble the layers and refrigerate before baking. When ready, bake covered and then uncovered to finish until golden.

→ What cheese combination works best here?

A blend of Gruyère and sharp cheddar provides rich flavor and melty texture that complements the sweet potatoes and onions.

→ Is there a way to thicken the creamy sauce?

Adding a small amount of gluten-free or all-purpose flour to the warm cream mixture before layering will gently thicken the sauce.

→ What herbs enhance this dish?

Dried thyme adds a subtle earthy note that pairs well with the sweetness of the potatoes and onions.

→ How should I garnish before serving?

Fresh chopped chives sprinkled on top bring a mild onion aroma and bright color contrast to the gratin.