Preheat your oven to 350°F (180°C).

Melt the butter in a medium saucepan over medium heat. Add the finely diced onion and cook until tender, about 3-5 minutes, stirring occasionally.

Stir in the minced garlic and chopped thyme. Cook for about 1 minute, until fragrant.

Sprinkle in the flour, stirring constantly, and cook until it turns lightly golden, roughly 1 minute.

Gradually whisk in milk (or heavy cream), cooking until the sauce thickens for about 1-2 minutes.

Remove from heat and fold in half of the shredded Gruyere cheese until melted. Season with nutmeg, salt, and pepper to taste.

In a greased baking dish, spread a layer of the cheese sauce, then layer the sliced potatoes. Repeat until you run out of potatoes, ending with sauce on top. Sprinkle the remaining cheese generously over the top.

Place in the preheated oven and bake for 45-60 minutes until tender and golden-brown.