Preheat your oven to 350°F (175°C) and grease a baking dish with butter.

Slice the potatoes into 1/8-inch thick rounds and keep them in cold water.

Melt 2 tablespoons of butter in a skillet, and sauté onion and garlic until translucent.

Add 2 tablespoons of flour, stir, then slowly whisk in 2 cups of milk and cook until thickened.

Remove from heat and stir in 1 cup of Gruyere cheese, nutmeg, salt, and pepper.

Layer half of the potato slices in the baking dish, pour half of the sauce over them, then repeat.

Sprinkle remaining Gruyere cheese on top and cover the dish with aluminum foil.

Bake for 45-60 minutes until fork-tender, then uncover for the last 15-20 minutes.

Let the gratin rest for about 10 minutes before serving.