Begin by melting 2 tablespoons of butter in a medium saucepan over medium heat. Once melted and bubbling, add 1 finely diced onion, cooking until it becomes transparent and tender, about 5 minutes. Stir in 2 minced garlic cloves and a teaspoon of fresh thyme leaves, allowing them to cook until fragrant, roughly 1 minute.

Sprinkle 2 tablespoons of flour into the onion mixture, whisking constantly for about 1-2 minutes until lightly golden, creating a roux. Gradually pour in 2 cups of milk while stirring continuously until the mixture thickens, approximately 5-7 minutes. Lower the heat and incorporate 2 cups of shredded Gruyere cheese, stirring until fully melted and smooth. Season with a pinch of nutmeg, salt, and pepper to taste.

Prepare a large baking dish by greasing it lightly with butter. Pour a thin layer of the cheese sauce at the bottom, just enough to cover the surface. Arrange half of the thinly sliced russet or Yukon gold potatoes in an even layer, overlapping slightly. Pour a portion of the cheese sauce over the potatoes, ensuring even coverage, and repeat with the remaining potatoes and sauce.

Preheat your oven to 350°F (180°C). Once layered, pour the remaining cheese sauce over the top layer of potatoes, ensuring every slice is coated. For an extra cheesy finish, sprinkle a generous amount of shredded cheese atop. Bake in the preheated oven for 45-60 minutes, or until the potatoes are tender and the top is golden and bubbly.

After baking, remove the dish from the oven and let it rest for about 10-15 minutes. This resting time allows the sauce to thicken and makes slicing easier. Once set, cut into portions and serve warm.