Cheesy Sweet Potato Gratin

The first time I made this gratin was for a dinner party where I completely forgot to buy regular potatoes, forcing me to use only sweet potatoes I had in the pantry. That accidental substitution turned into the most requested dish at every gathering since. The way the natural sweetness plays against the savory cheese and onions creates something magical that people just cannot stop talking about.

Last Thanksgiving my sister-in-law took one bite and literally stopped mid-conversation, her eyes widening as she reached for her phone to snap a picture before even touching anything else on her plate. Watching people discover that layer of caramelized onions hiding between the potatoes has become one of my favorite kitchen moments ever.

Ingredients

• 2 lbs sweet potatoes peeled and thinly sliced: The thinner you slice these the more elegant layers you will create, and a mandoline makes this so much faster than hand cutting
• 2 large yellow onions thinly sliced: These need patience to properly caramelize but that deep golden color and sweetness is absolutely worth every minute of stirring
• 2 cups shredded Gruyère cheese: This melts into the most gorgeous creamy pools and adds that nutty sophisticated flavor that makes the dish feel special
• 1 cup shredded sharp cheddar cheese: The sharpness cuts through the sweet potatoes and cream beautifully while adding those perfect golden cheesy edges on top
• 1 1/2 cups heavy cream: Do not substitute this because the high fat content is what transforms the potatoes into silk rather than mushy layers
• 1/2 cup whole milk: This lightens the cream just enough so the dish feels rich but not overwhelmingly heavy
• 3 tbsp unsalted butter: Use this for caramelizing the onions and mixing into the cream mixture because butter makes everything taste better
• 1/2 tsp dried thyme: This herb pairs perfectly with both the sweet potatoes and the cheese without overpowering either flavor
• 1/4 tsp ground nutmeg: Just a pinch adds this warmth that you cannot quite identify but makes the whole dish taste more complex and professional

Instructions

Caramelize the onions:

Melt 2 tablespoons butter in a large skillet over medium heat, add the sliced onions with a pinch of salt, and cook them slowly for 20-25 minutes while stirring frequently until they turn a deep golden brown. Add the minced garlic during the last minute so it becomes fragrant without burning.

Warm the cream mixture:

In a small saucepan, combine the heavy cream, whole milk, dried thyme, nutmeg, black pepper, and remaining 1 tablespoon butter. Heat over medium-low until steaming hot but not boiling, and whisk in the flour if you are using it for a slightly thicker sauce.

Build the layers:

Arrange half the sweet potato slices in the bottom of your prepared baking dish and sprinkle them lightly with salt. Spread half the caramelized onions over the potatoes, then sprinkle with half of both the Gruyère and cheddar cheeses.

Repeat and pour:

Add the remaining sweet potatoes, then the rest of the onions, and finish with all the remaining cheese on top. Pour that warm cream mixture evenly over everything so it seeps down into all the layers.

Bake to perfection:

Cover tightly with foil and bake for 40 minutes, then remove the foil and continue baking for another 20 minutes until the top is bubbling and golden. Let it rest for 10 minutes before serving so the sauce has time to set slightly.

Pin Recipe

This dish has officially become the thing my friends request for every birthday dinner and holiday gathering. Something about that first forkful cutting through all those cheesy layers just makes people feel incredibly cared for and comforted.

Making It Ahead

You can assemble the entire gratin up to 24 hours before baking and keep it covered in the refrigerator. When you are ready to bake, add about 10 extra minutes to the covered baking time since it will be cold going into the oven.

Perfect Pairings

A crisp white wine like Sauvignon Blanc cuts through the richness beautifully, or if you prefer red, a light Pinot Noir works wonderfully without overwhelming the delicate flavors. For a complete meal, serve alongside roasted chicken or a simple green salad with tangy vinaigrette.

Serving Suggestions

Fresh chives sprinkled right before serving add this beautiful pop of color and mild onion flavor that brightens the whole dish. For special occasions, a handful of crispy fried onions on top creates this incredible crunch contrast.

• Let guests see those gorgeous layers by serving straight from the baking dish
• Warm individual plates for 5 minutes so the cheese stays melty longer
• Extra passing salt at the table lets people adjust to their taste

Pin Recipe

Every time I pull this golden bubbling dish out of the oven and catch that first whiff of caramelized onions and melted cheese, I remember why some dishes become classics. Hope this becomes one of those recipes your family asks for again and again.

Recipe FAQs

→ How do I caramelize onions perfectly?

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

→ Can I use other cheeses in the gratin?

Yes, Gruyère and sharp cheddar provide rich flavor, but you can substitute with fontina, mozzarella, or gouda for different textures and tastes.

→ How do I ensure the sweet potatoes are tender when baked?

Slice them thinly and bake covered with foil initially to trap steam, then uncovered to brown the top, resulting in soft, fork-tender layers.

→ Is gluten-free flour necessary for thickening?

The flour is optional; it helps slightly thicken the cream mixture but can be omitted for a looser texture or if avoiding gluten.

→ What are good beverage pairings for this gratin?

A crisp white like Sauvignon Blanc or a light red such as Pinot Noir complements the creamy, cheesy layers and sweet onion notes.