Preheat your oven to 350°F (175°C).

Melt 2 tablespoons of butter over medium heat and sauté 1 finely chopped onion and 2 minced garlic cloves until translucent, about 5 minutes.

Stir in 1 tablespoon of all-purpose flour and cook for about a minute. Gradually whisk in 2 cups of heavy cream and ½ cup of milk, stirring until thickened.

Add in 1 teaspoon of dried thyme, ¼ teaspoon of nutmeg, and salt and pepper to taste. Remove from heat.

In a greased baking dish, layer half of the sliced potatoes. Pour half of the cream sauce over them and sprinkle with half of the Gruyère cheese.

Repeat with remaining potatoes, cream sauce, and Gruyère cheese. Add additional cheese on top.

Cover with foil and bake for 50 minutes.

Uncover and bake for another 20 minutes until bubbly and golden brown.

Cool for 10 minutes before serving.