Method:
Prepare the sweet potatoes: Wash and peel the sweet potatoes, then cut them into thin slices, about 3-4mm thick. You can use a mandolin to obtain uniform slices.
Prepare the gratin base: Lightly grease a baking dish with olive oil and rub it with a clove of garlic cut in half (optional). Layer the sweet potato slices on the bottom of the baking dish.
Add the cheese: Sprinkle some of the crumbled or chopped goat cheese over the potatoes. Add some chopped hazelnuts and a sprinkle of grated parmesan.
Layer: Continue layering the sweet potatoes, goat cheese, hazelnuts and parmesan until you have used up all the ingredients, finishing with a layer of cheese and hazelnuts on top.
Make the cream: In a bowl, mix the cream with a pinch of salt, pepper, nutmeg (if using) and a few leaves of rosemary or thyme. Pour the cream evenly over the gratin.
Bake: Preheat the oven to 350°F. Cover the baking dish with aluminum foil and bake the gratin for 30 minutes. Remove the foil and continue baking for another 15-20 minutes, or until the sweet potatoes are soft and the top is golden and crispy.
Final gratin: If you want a crispier crust, turn on the oven grill in the last few minutes of cooking.
Serve: Let the gratin rest for a few minutes before serving, so that the flavors blend well.
