Creating Potato Gratin can be a straightforward process if you follow these steps:

1. Preheat the Oven: Preheat your oven to 375°F (190°C).
2. Prepare the Baking Dish: Grease a large baking dish with butter to prevent sticking.
3. Slice the Potatoes: Wash and peel the potatoes. Slice them into thin, uniform rounds using a mandoline or sharp knife. Set aside.
4. Make the Cream Mixture: In a mixing bowl, combine heavy cream, minced garlic, sliced onion, thyme, salt, pepper, and nutmeg. Mix well.
5. Layer the Potatoes: Start layering the potato slices in the greased baking dish, slightly overlapping them.
6. Add Cheese: Sprinkle a layer of Gruyère and cheddar cheeses over the potatoes.
7. Pour the Cream Mixture: Repeat layering potatoes and cheese, then pour the cream mixture evenly over the entire dish.
8. Top with Butter: Dot the top with small pieces of butter for added richness.
9. Cover and Bake: Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 30-40 minutes.
10. Uncover and Brown: After the initial baking time, remove the foil and keep baking for an additional 15-20 minutes until the top is golden and bubbly.
11. Cool: Once it’s finished, take the gratin from the oven and let it cool for about 10-15 minutes before serving.
12. Garnish: Before serving, garnish with fresh parsley if desired.

By following these simple instructions, you will create an incredible Potato Gratin that is sure to impress!