4 pounds sweet potatoes, peeled and sliced into 1/8 inch rounds
2 cups heavy cream
3 cloves garlic, minced
1 teaspoon fresh thyme, finely chopped
1 teaspoon fresh rosemary, finely chopped
1/2 teaspoon ground nutmeg
1 teaspoon salt
1/2 teaspoon cracked black pepper
2 cups shredded Gruyere cheese
1/2 cup grated parmesan cheese
2 tablespoons butter, for greasing the baking dish
Instructions
Preheat your oven to 375 degrees Fahrenheit. Generously grease a 9×13 inch ceramic or glass baking dish with the softened butter, ensuring you cover the bottom and all the sides to prevent sticking.
Peel the sweet potatoes and slice them into thin, even rounds about 1/8 inch thick. Using a mandoline slicer is highly recommended for consistency, which ensures that all the slices cook at the same rate. Set the slices aside in a large bowl.
In a medium saucepan over medium heat, combine the heavy cream, minced garlic, chopped thyme, rosemary, nutmeg, salt, and pepper. Heat the mixture until it just begins to simmer, then remove it from the heat. This allows the herbs and garlic to infuse the cream with flavor before baking.
Arrange about one third of the sweet potato slices in the bottom of the prepared baking dish, overlapping them slightly. Sprinkle with one third of the Gruyere cheese. Repeat the layers two more times, ending with a final layer of potatoes. Pour the warm cream mixture evenly over the top of the potatoes.
Cover the baking dish tightly with aluminum foil. Place it in the oven and bake for 40 to 45 minutes. Covering the dish allows the potatoes to steam and soften without the top burning or the cream evaporating too quickly.
Remove the foil and sprinkle the remaining parmesan cheese over the top. Return the dish to the oven, uncovered, and bake for another 15 to 20 minutes. The dish is finished when the potatoes are fork-tender and the top is golden brown and bubbling. Let it rest for 10 minutes before serving to allow the sauce to thicken.
