Cheesy Sweet Potato Gratin

The first time I made this gratin was for a Friendsgiving that stretched until midnight. Someone brought cheap wine and everyone ended up sitting on the kitchen floor. That sweet and savory combination disappeared faster than the main dish, and people are still asking me about it three years later.

I learned the hard way that sweet potatoes vary wildly in thickness. My mandoline was acting up once and I did the slices by hand. The uneven layers actually created gorgeous texture contrast, with some edges getting crispy while others stayed meltingly tender.

Ingredients

• 2 lbs sweet potatoes, peeled and thinly sliced: Try to slice them as evenly as possible for uniform cooking. A mandoline saves time here.
• 2 large yellow onions, thinly sliced: These need time to caramelize properly. Do not rush this step. The deep sweetness is what makes the dish.
• 2 cloves garlic, minced: Add this at the very end of caramelizing so it does not burn.
• 2 cups shredded Gruyère cheese: Gruyère melts beautifully and adds that nutty depth.
• 1 cup shredded sharp cheddar cheese: The sharpness cuts through the rich cream sauce.
• 1 1/2 cups heavy cream: Do not use half and half. You need the fat content for proper texture.
• 1/2 cup whole milk: This slightly lightens the cream without sacrificing richness.
• 3 tbsp unsalted butter: 2 tbsp for the onions and 1 tbsp for the cream mixture.
• 1/2 tsp dried thyme: Fresh thyme works too if you have it.
• 1/2 tsp ground black pepper: Freshly ground makes a noticeable difference.
• 1/2 tsp salt: Plus more to taste throughout the process.
• 1/4 tsp ground nutmeg: Just a whisper of warmth that makes people ask what the secret ingredient is.
• 1 tbsp gluten-free flour: Optional but helps stabilize the sauce.
• 2 tbsp chopped fresh chives: For that pop of color and fresh onion flavor at the end.

Instructions

Caramelize the onions:

Melt 2 tbsp butter in a large skillet over medium heat. Add the sliced onions with a pinch of salt. Cook them slowly, stirring often, until they turn deep golden brown. This takes 20 to 25 minutes and cannot be rushed. Add the minced garlic during the last minute.

Prepare the cream base:

Heat the cream, milk, thyme, nutmeg, pepper, and remaining 1 tbsp butter in a small saucepan over medium-low heat. Bring it just to a steaming point. Do not let it boil. If using flour, whisk it in now until smooth.

Build the layers:

Layer half the sweet potato slices in your prepared baking dish. Give them a light sprinkle of salt. Spread half the caramelized onions on top, then half the cheeses. Repeat everything with the remaining ingredients.

Pour and cover:

Pour the warm cream mixture evenly over all the layers. Cover the dish tightly with foil to trap the steam.

Bake covered:

Bake at 375°F for 40 minutes. The foil keeps everything moist while the potatoes cook through.

Finish uncovered:

Remove the foil and bake another 20 minutes until the top is golden and bubbly. You want those cheese edges to get a little crispy.

Rest and serve:

Let the gratin rest for 10 minutes before serving. This helps the sauce set slightly so it does not run everywhere when you scoop it.

Save

This became my go-to for difficult dinner parties because it feels fancy but is almost entirely hands-off during baking. My sister requested it for her holiday meal instead of the usual marshmallow-topped casserole, and even the skeptical relatives went back for seconds.

Make-Ahead Magic

You can assemble this entire gratin a day before baking. Keep it covered in the refrigerator. Add about 10 minutes to the covered baking time if it is cold from the fridge. The flavors actually develop overnight, making it even better.

Serving Suggestions

This rich dish needs something bright and acidic alongside. A simple arugula salad with lemon vinaigrette cuts through the cream beautifully. Roasted green beans or asparagus also work well. For wine, that crisp white or light red suggestion in the notes is spot on.

Customization Ideas

Sometimes I add a layer of sautéed spinach between the potato layers for color and nutrients. A pinch of smoked paprika in the cream mixture adds subtle depth that people notice but cannot quite place.

• Try swapping half the sweet potatoes for Yukon Golds if you want a mixed potato effect
• Add fresh sage leaves between layers for a more autumnal flavor profile
• Top with toasted breadcrumbs during the last 10 minutes for extra crunch

Save

There is something deeply satisfying about pulling that golden, bubbling dish from the oven. The smell of caramelized onions and melting cheese fills the whole house. It is comfort food that feels like a special occasion.

Recipe FAQs

→ How do I caramelize the onions effectively?

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

→ Can I make this gratin gluten-free?

Yes, using gluten-free flour in the cream mixture ensures it's gluten-free. Just verify any cheese labels for additives.

→ What cheeses work best for this dish?

A blend of Gruyère and sharp cheddar provides a perfect balance of creaminess and tang that melts beautifully in this gratin.

→ How should I layer the ingredients?

Alternate layers of sweet potato slices, caramelized onions, and the cheese blend. Pour the warm cream sauce over before baking.

→ What are good wine pairings for this gratin?

Try a crisp Sauvignon Blanc or a light-bodied Pinot Noir to complement the rich, savory flavors.

→ Can this dish be prepared ahead of time?

Yes, you can assemble in advance and gently reheat before serving to maintain freshness and flavor.