If your Thanksgiving sweet potatoes still come with marshmallows, this is your sign to change that this year. This gratin is savory, cheesy, and a little bit Italian.
Thin layers of sweet potato and butternut squash bake in a rosemary-garlic milk with plenty of Parmigiano Reggiano. It's mostly milk and stock instead of all cream, so it's much lighter than a classic gratin.
My favorite part is what happens halfway through. After the covered bake, there's a pool of liquid at the bottom of the dish. Instead of tossing it, I whisk it into a quick béchamel-style sauce and spoon it back over the top. All that flavor goes right back into the dish, and it bakes up creamy and golden without loading it with cream.
Let's get cookin'!
INGREDIENTS
Serves 8-10
FOR THE GRATIN
2 lbs sweet potatoes
1 medium butternut squash, peeled and seeded
1 cup whole milk
½ cup heavy cream
3 garlic cloves (2 smashed, 1 halved)
1 tbsp fresh rosemary, finely chopped
¼ tso freshly grated nutmeg
1 ½ tsp kosher salt
½ tsp black pepper
1 ½ cup finely grated Parmesan cheese
1 tbsp olive oil
BÉCHAMEL-STYLE SAUCE
2 tbsp unsalted butter
2 tbsp cups all-purpose flour
Liquid from the bottom of the baked gratin
¼ cup finely grated Parmesan cheese
Instructions
Prep the dish
Heat the oven to 375°F. Rub a 2 to 3 quart baking dish with the cut side of the halved garlic clove, then coat it with the olive oil.Infuse the milk
In a small pot, warm the milk and cream with the smashed garlic, rosemary, nutmeg, salt, and pepper. Bring it to a gentle simmer, then turn off the heat and let it steep for 5 minutes while you slice.Slice the vegetables
Peel the sweet potatoes and slice them about ⅛ inch thick. Cut the squash neck into ¼ inch rounds. Halve the round bottom of the squash, scoop out the seeds, and cut it into ¼ inch half-moons. The squash is cut a little thicker because it softens faster.Layer
Remove the smashed garlic from the milk. Shingle a third of the slices into the dish, alternating sweet potato and squash. Pour over a third of the milk and sprinkle with ½ cup of the Parmigiano. Repeat twice, ending with cheese on top.Bake covered
Cover tightly with foil and bake for 35 to 40 minutes, until the slices are tender when pierced with a knife.Make the sauce
Carefully tilt the dish and pour the liquid from the bottom into a measuring cup. In a small saucepan over medium heat, melt the butter and whisk in the flour for about 1 minute. Slowly whisk in the reserved liquid and cook for 2 to 3 minutes, until it thickens. Stir in the ¼ cup of Parmigiano, then spoon the sauce evenly over the gratin.Bake uncovered
Return the dish to the oven, uncovered, and bake for 25 to 30 minutes, until the top is deep golden and bubbling.Rest and serve
Return the dish to the oven, uncovered, and bake for 25 to 30 minutes, until the top is deep golden and bubbling.
Jo’s Tips
Use a mandoline. Even slices cook evenly. If you're slicing by hand, go slow and keep them as thin as you can.
Don’t skip the rest. The sauce keeps thickening as it cools, so a short rest gives you creamy layers instead of a runny scoop.
Make it ahead. Slice the sweet potatoes the night before and store them covered in water in the fridge. Pat them very dry before layering. You can also bake the whole gratin a day ahead, refrigerate it, and reheat it covered at 350°F for about 25 minutes, then uncover for the last 5.
STORAGE & REHEATING
To store: Refrigerate within 2 hours, covered or in an airtight container, for 3 to 4 days.
Freezing: I suggest not freezing it, since the sauce can turn grainy once it thaws.
Reheating: warm the whole dish covered at 350°F for 20 to 25 minutes, or microwave a single portion at 50% power for 1 to 2 minutes. A splash of milk helps if it looks dry.
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