Preheat your oven to 350°F (180°C). Gather your ingredients and equipment.

Melt 4 tablespoons of butter in a large saucepan over medium heat. Add one finely chopped onion and sauté until tender.

Stir in 2 minced garlic cloves and fresh thyme, cooking for an additional minute until fragrant.

Sprinkle in 1/4 cup of flour, stirring constantly for about a minute until it turns lightly golden.

Gradually pour in 2 cups of milk, whisking continuously as you cook until the sauce thickens.

Remove from heat and stir in 1.5 cups of grated Gruyère cheese until melted. Season with salt, pepper, and nutmeg.

In a greased baking dish, pour a portion of the cheese sauce and layer with sliced potatoes, repeating until the dish is full.

Cover the last layer with remaining cheese sauce and sprinkle extra cheese on top.

Bake for 45–60 minutes, until potatoes are tender and top is golden brown. Broil for 3–5 minutes for a crisp finish.

Allow to rest for about 10 minutes before serving.