Cheesy Sweet Potato Gratin

The kitchen was filled with this incredible sweet aroma that had my roommate wandering in from the living room, asking what smelled like heaven. I was experimenting with combining sweet potatoes and caramelized onions, not sure if the flavors would work together. When that first bubbling dish came out of the oven, golden and fragrant, we both stood there just watching it, knowing this was going to be special.

Last Thanksgiving, my sister claimed she was not hungry until this gratin came out of the oven. She took one hesitant bite, then promptly announced she was making room for seconds and even asked to take home the leftovers. Now every family gathering, someone asks if I am bringing those potatoes.

Ingredients

• 2 lbs sweet potatoes: I slice these as thin as possible because they cook more evenly and create this beautiful layered effect that looks impressive
• 2 large yellow onions: Take your time caramelizing them properly because that deep golden color develops natural sweetness you cannot rush
• 2 cloves garlic: Add this at the very end so it does not burn and become bitter
• 2 cups shredded Gruyère cheese: This melts beautifully and adds that nutty, sophisticated flavor that makes the dish feel special
• 1 cup shredded sharp cheddar cheese: The sharpness cuts through the rich cream and balances the sweet potatoes perfectly
• 1 1/2 cups heavy cream and 1/2 cup whole milk: I use this combination because it is rich without being overwhelming
• 3 tbsp unsalted butter: One tablespoon goes into the cream mixture and two tablespoons help caramelize the onions
• 1/2 tsp dried thyme: This herb has an earthy quality that bridges the gap between the sweet and savory elements
• 1/2 tsp ground black pepper: Freshly cracked gives you these little bursts of spice throughout
• 1/2 tsp salt plus more to taste: Sweet potatoes need enough salt to really shine
• 1/4 tsp ground nutmeg: Just a tiny pinch adds warmth that people notice but cannot quite identify
• 1 tbsp flour: This optional thickener helps the cream coating cling to every single layer
• 2 tbsp chopped fresh chives: These add a fresh pop of color and mild onion flavor right at the end

Instructions

Heat the oven and prepare your dish:

Set your oven to 375°F and butter a 9x13 baking dish thoroughly because you do not want any sticking.

Caramelize the onions slowly:

Melt 2 tablespoons butter in a large skillet and cook the sliced onions with a pinch of salt for 20 to 25 minutes until they turn this deep golden color.

Add the garlic:

Toss in the minced garlic for just one minute until fragrant, then remove everything from the heat.

Warm the cream mixture:

In a small saucepan, heat the cream, milk, thyme, nutmeg, pepper, remaining butter, and optional flour until steaming but not boiling.

Start layering:

Arrange half the sweet potatoes in the baking dish and sprinkle them lightly with salt.

Add the first layer of flavor:

Spread half the caramelized onions over the potatoes, then top with half the Gruyère and cheddar.

Build the second layer:

Repeat with the remaining sweet potatoes, onions, and cheese, creating this beautiful strata that will melt together.

Pour over the cream:

Drizzle the warm cream mixture evenly over everything, watching it seep down into all the layers.

Bake covered first:

Cover tightly with foil and bake for 40 minutes so the potatoes cook through in all that steam.

Get that golden top:

Remove the foil and bake for 20 more minutes until the top is bubbly and golden brown.

Let it rest:

Wait 10 minutes before serving because this helps the layers set and makes serving much easier.

Add the finishing touch:

Sprinkle fresh chives over the top right before serving for that restaurant style finish.

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My neighbor smelled this cooking through our open windows and actually knocked on my door to ask what I was making. We ended up sharing dinner on her back patio, and she has requested this recipe three times since then.

Make It Your Own

I have discovered that adding sautéed mushrooms between the layers adds this wonderful earthy flavor that makes the dish even more substantial. A pinch of smoked paprika in the cream mixture gives this subtle smoky undertone that keeps people guessing.

Timing Is Everything

You can caramelize the onions up to two days ahead and store them in the refrigerator, which actually concentrates their flavor even more. The whole gratin assembles beautifully the night before and just needs a few extra minutes in the oven if baking cold from the fridge.

Serving Suggestions

This gratin pairs surprisingly well with a crisp white wine like Sauvignon Blanc that cuts through the richness. I also love serving it alongside roast chicken or even as a main dish with a simple green salad when I want something comforting but not too heavy.

• Reheat leftovers gently covered so the creamy sauce does not separate
• Use a mandoline for perfectly even slices that cook at the same rate
• Add a sprinkle of extra cheese during the last 5 minutes if you want an extra golden crust

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There is something deeply satisfying about serving a dish that looks this elegant but comes together with such simple ingredients and straightforward techniques.

Recipe FAQs

→ How do I caramelize onions properly?

Cook sliced onions slowly over medium heat with a pinch of salt, stirring frequently until they turn deep golden brown, usually about 20-25 minutes. This brings out their natural sweetness.

→ Can I substitute the cheeses used?

Yes, Gruyère and sharp cheddar provide a nice balance of creaminess and sharpness, but you can use similar melting cheeses like fontina or mozzarella depending on preference.

→ What is the purpose of the cream and milk mixture?

It adds richness and moisture to gently cook the sweet potatoes during baking, creating a smooth and creamy texture throughout the gratin.

→ How do I ensure the sweet potatoes cook evenly?

Slice the sweet potatoes thinly and evenly, then layer them uniformly in the baking dish. Baking covered first helps steam and soften the potatoes before crisping the top uncovered.

→ Can this dish be prepared ahead of time?

Yes, it can be assembled and stored refrigerated, then gently reheated before serving to maintain flavors and texture.