Creating Sweet Potato Gratin is simple if you follow these easy steps:

1. Preheat the Oven: Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking dish with butter.
2. Prepare Sweet Potatoes: Peel and slice the sweet potatoes into thin rounds, about 1/8 inch thick.
3. Make Cream Mixture: In a bowl, combine the heavy cream, minced garlic, thyme, salt, pepper, and nutmeg. Stir until well mixed.
4. Layer Sweet Potatoes: Place a layer of sweet potato slices at the bottom of the prepared baking dish.
5. Add Cream: Pour a portion of the cream mixture over the sweet potatoes, followed by a sprinkle of Gruyère cheese.
6. Repeat Layers: Continue layering the sweet potatoes, cream mixture, and Gruyère cheese until all ingredients are used, finishing with a layer of sweet potatoes and cream on top.
7. Top with Parmesan: Sprinkle the grated Parmesan cheese evenly over the top layer.
8. Cover and Bake: Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes.
9. Uncover and Brown: After 30 minutes, remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.
10. Rest Before Serving: Once baked, allow the gratin to rest for about 10 minutes before serving. This will help the layers set.

These steps will guide you in creating this incredible dish effortlessly.