Cheesy Sweet Potato Gratin

The smell of caramelizing onions always pulls me into the kitchen, no matter what else I'm doing. I first made this gratin on a rainy Sunday when I needed something warm and comforting to fill the house with delicious aromas. My roommate wandered in halfway through, asking what smelled so incredible, and ended up staying for dinner.

I served this at my first proper dinner party last fall, nervously checking the oven every five minutes. When I finally pulled it out, bubbling and golden with that incredible cheese crust forming, I knew it was going to be okay. My friend Sarah actually asked for the recipe before she even finished her first serving.

Ingredients

• 2 lbs sweet potatoes, peeled and thinly sliced: I've learned that using a mandoline gives you those perfectly even slices that cook uniformly, but a sharp knife and steady hand works just fine too
• 2 large yellow onions, thinly sliced: Take your time caramelizing these properly—those deep golden brown layers are where all the complex flavor develops
• 2 cloves garlic, minced: Add this in the last minute of caramelizing so it doesn't burn and turn bitter
• 2 cups shredded Gruyère and 1 cup sharp cheddar: The Gruyère brings that nutty sophistication while cheddar adds the familiar sharpness we all crave
• 1 1/2 cups heavy cream and 1/2 cup whole milk: This ratio creates the perfect velvety consistency without being overwhelmingly rich
• 3 tbsp unsalted butter: Reserve one tablespoon for the cream mixture and use the rest for caramelizing those onions
• 1/2 tsp dried thyme, 1/4 tsp nutmeg, 1/2 tsp black pepper, and 1/2 tsp salt: The thyme and nutmeg are subtle but essential—don't skip them
• 1 tbsp gluten-free flour: This optional thickener helps the sauce cling to every single layer
• 2 tbsp chopped fresh chives: These add a fresh pop of color and mild onion flavor that brightens the whole dish

Instructions

Get your oven and dish ready:

Preheat to 375°F and butter a 9x13-inch baking dish thoroughly—I learned the hard way that cheese sticks to everything

Caramelize the onions:

Melt 2 tablespoons butter in a large skillet over medium heat, add onions with a pinch of salt, and cook patiently for 20-25 minutes until they turn deep golden brown

Add the aromatic finish:

Stir in the minced garlic during that final minute of cooking, then remove everything from the heat

Warm the cream mixture:

Heat cream, milk, thyme, nutmeg, pepper, remaining butter, and flour if using in a small saucepan until steaming but not boiling

Build the first layer:

Arrange half the sweet potato slices in the prepared dish, season lightly, then spread half the caramelized onions over top

Add the cheese:

Sprinkle half of both cheeses over the onion layer, making sure to distribute it evenly

Repeat and complete:

Layer remaining potatoes, onions, and cheese, then pour that warm cream mixture slowly and evenly over everything

Cover and bake:

Cover tightly with foil and bake for 40 minutes—the steam helps cook those sweet potatoes through

Create the golden crust:

Remove foil and bake another 20 minutes until the top is bubbly and beautifully browned

Let it rest:

Wait 10 minutes before serving—this seems impossible but helps the layers set so you get neat slices instead of a delicious mess

Save

This recipe became my go-to for bringing to friends' houses after I made it for Thanksgiving three years running. My aunt finally asked for the recipe last year, admitting she'd been trying to recreate it based on memory ever since that first Thanksgiving.

Make It Ahead

I've discovered you can assemble everything up to 24 hours in advance, cover tightly, and refrigerate. Just add 10-15 minutes to the covered baking time if you're starting from cold.

Perfect Wine Pairings

A crisp Sauvignon Blanc cuts through the richness beautifully, or if you prefer red, a light Pinot Noir won't overpower the delicate flavors.

Serving Suggestions

This gratin works wonderfully alongside roasted chicken or pork tenderloin. It's substantial enough to stand alone as a main dish with a simple green salad.

• Let guests add their own fresh chives at the table for a fun interactive element
• A small dollop of sour cream on top adds a tangy contrast
• 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 but comes from such humble ingredients. Enjoy every bubbling, cheesy bite.

Recipe FAQs

→ How do you caramelize onions for this dish?

Cook sliced onions slowly in butter over medium heat with a pinch of salt until deeply golden and soft, about 20-25 minutes, stirring frequently to prevent burning.

→ Can I make this gratin ahead of time?

Yes, it can be assembled in advance and reheated gently before serving to maintain its creamy texture.

→ What cheeses work best in this gratin?

A combination of Gruyère and sharp cheddar provides a rich, nutty flavor and creamy melt that complement the sweet potatoes.

→ Is this dish gluten-free?

It is gluten-free if you use gluten-free flour for thickening and ensure the cheeses do not contain additives with gluten.

→ What wine pairs well with this gratin?

Crisp white wines like Sauvignon Blanc or light reds such as Pinot Noir balance the richness of the gratin nicely.