3 pounds sweet potatoes, peeled and sliced into 1/8 inch rounds
2 cups heavy cream
1 cup whole milk
3 cloves garlic, minced
1 tablespoon fresh thyme leaves
1 tablespoon fresh rosemary, finely chopped
1 cup shredded Gruyere cheese
1/2 cup grated Parmesan cheese
1 teaspoon salt
1/2 teaspoon black pepper
2 tablespoons butter, for greasing the baking dish
Instructions
Preheat your oven to 375 degrees Fahrenheit. Generously grease a 9 by 13 inch baking dish with the butter to ensure the potatoes do not stick and the edges get nicely crisped.
Peel the sweet potatoes and use a sharp knife or a mandoline to slice them into even rounds about 1/8 inch thick. Keeping the slices uniform is key to making sure every piece of potato cooks through at the same rate.
In a medium saucepan, combine the heavy cream, whole milk, minced garlic, thyme, and rosemary. Heat the mixture over medium low heat until it just begins to simmer, then remove it from the heat and stir in the salt and black pepper.
Place a layer of sweet potato slices in the bottom of the prepared baking dish, overlapping them slightly. Sprinkle a portion of the Gruyere and Parmesan cheese over the layer. Repeat this process until all potatoes and most of the cheese are used, finishing with a layer of potatoes.
Carefully pour the warm cream mixture over the layered potatoes, making sure it seeps down into the gaps. Sprinkle the remaining cheese over the top of the dish to create a delicious crust during the baking process.
Cover the dish with aluminum foil and bake for 30 minutes. Remove the foil and continue baking for another 25 to 30 minutes, or until the potatoes are tender when pierced with a fork and the top is bubbly and golden brown. Let the dish rest for 10 minutes before serving to allow the sauce to thicken.
