Melt 2 tablespoons of butter in a saucepan over medium heat. Add one chopped onion and sauté for 3-5 minutes until tender. Stir in 2 minced garlic cloves and 1 teaspoon of thyme, cooking for 1 minute until fragrant.

Sprinkle in 2 tablespoons of flour and cook for 1-2 minutes. Gradually whisk in 2 cups of milk and stir until thickened, about 1-2 minutes.

Reduce heat to low, add 1 cup of Gruyere cheese, and stir until melted. Season with nutmeg, salt, and pepper.

Spread a thin layer of cheese sauce in a greased baking dish. Layer half of thinly sliced potatoes, pour some sauce over, and repeat with remaining potatoes and sauce.

Cover with foil and bake at 350°F (180°C) for 45-60 minutes until potatoes are tender.

For a crispy top, remove foil in the last 10 minutes of baking, and broil for an additional 2-3 minutes while watching to prevent burning.