Preheat your oven to 350°F (175°C). Adjust the oven rack to the center position.

Thinly slice the potatoes into 1/8-inch rounds and place them in a bowl of cold water.

Mince the onion, grate the garlic, and chop the fresh thyme.

Melt 4 tablespoons of unsalted butter in a medium saucepan over medium heat. Add the onion, garlic, and thyme, and sauté for 3–5 minutes.

Sprinkle 3 tablespoons of flour over the sautéed mixture, stirring continuously for about 1 minute to form a roux.

Whisk in 2 cups of whole milk gradually, stirring until the sauce thickens, about 5–7 minutes.

Remove the saucepan from heat and stir in 1/2 cup of grated gruyere cheese until fully melted.

In a buttered baking dish, arrange half of the potato slices on the bottom. Pour half of the cheese sauce over the potatoes.

Repeat the layering with the remaining potatoes and cheese sauce.

Sprinkle any remaining cheese on top of the last layer.

Cover with aluminum foil and bake for 45–60 minutes, removing the foil during the last 15-20 minutes.

Let the gratin rest for 10 minutes before serving.