Preheat the Oven:

o Preheat your oven to 375°F (190°C).

o Coat a 9x13-inch baking dish with butter or a non-stick spray to prevent sticking.

Prepare the Cream Sauce:

o In a medium saucepan, melt the butter over medium heat.

o Add the finely diced onion and cook until softened, about 4-5 minutes.

o Add the minced garlic and thyme, and cook for another minute until fragrant.

Make the Roux:

o Dust the onions and garlic with flour, ensuring an even coating.

o Stir well to combine, cooking for about 1 minute until the flour turns a light golden color.

Add the Milk:

o Slowly add the milk while continuously whisking to avoid any lumps.

o Keep cooking the sauce, stirring often, until it reaches a thicker consistency, which should take around 3 to 4 minutes.

Incorporate the Cheese:

o Reduce the heat to low and slowly add the shredded Gruyère cheese to the sauce, stirring until the cheese is melted and the sauce is smooth.

o Incorporate a dash of nutmeg (if desired) and adjust the flavor with salt and pepper to your liking.

o Remove from heat.

Layer the Potatoes:

o Arrange half of the sliced potatoes evenly in the prepared baking dish.

o Drizzle half of the cheese sauce over the potatoes, ensuring it is distributed uniformly.

o Layer the remaining potatoes on top, and finish by pouring the rest of the cheese sauce over them, making sure all the potatoes are covered.

Bake:

o Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and continue baking for an additional 30-40 minutes, or until the potatoes are tender and the top is golden brown and bubbling.

Rest and Serve:

o Let the Potato Gratin rest for 10 minutes before serving to allow the sauce to set. Serve warm as a delicious side dish with your favorite main course.