Golden and Sweet Potato Gratin

6 tablespoons unsalted butter, at room temperature
2 cups milk
3 garlic cloves, pressed
2 teaspoons kosher salt
1 tablespoon fresh thyme leaves
1/2 teaspoon pepper
1/8 teaspoon grated nutmeg
2¼ pounds Yukon gold potatoes, well-scrubbed
1½ pounds dark-skinned, sweet potatoes, peeled
1 cup heavy cream
1 cup grated parmesan, optional

Preheat the oven to 400°F.

Spread 2 tablespoons of the butter over the bottom and sides of a 13 x 11-inch baking dish.

Combine the milk, garlic, salt, thyme, pepper and nutmeg in a heavy, medium-sized saucepan and bring just to a boil.

Thinly slice the potatoes and the sweet potatoes and place them in the prepared dish. Pour the hot milk mixture over the potatoes. Sprinkle the mixture with the cheese, if using. Dot the top with 2 tablespoons of the butter.

Cover the dish tightly with aluminum foil and bake until the potatoes are tender and the milk is almost completely absorbed, 35-40 minutes.

Meanwhile bring the cream to a boil in a small saucepan. Remove the foil from the baking dish and pour the cream over the potatoes. Dot the top with the remaining butter and continue baking uncovered until teh cream is very thick and the top of the gratin is golden brown, about 30-35 minutes. 

Cool slightly and serve.


- Recipe courtesy of Jeanne Kelly's book Blue Eggs and Yellow Tomatoes