Prepare the Sauce:
Melt the butter in a saucepan over medium heat. Add the diced onion and sauté until soft and translucent, about 3-5 minutes.
Stir in the garlic and thyme, cooking for another minute until fragrant.
Sprinkle the flour over the mixture, stirring well. Cook for about a minute until it turns a light golden color.
Gradually pour in the milk, stirring constantly until the sauce thickens, about 1-2 minutes.
Incorporate the Cheese:
Lower the heat and add 1/2 cup of the shredded Gruyere cheese. Stir until the cheese melts completely into the sauce.
Season the sauce with a pinch of nutmeg (if using), and adjust with salt and pepper to your taste.
Assemble the Dish:
Spread a small amount of the sauce over the bottom of a large baking dish or cast iron pan.
Arrange the sliced potatoes in layers, adding a spoonful of sauce between each layer. For extra creaminess, dip each potato slice into the sauce before layering.
Pour any remaining sauce over the top of the potatoes, ensuring they are well-coated. Sprinkle the rest of the Gruyere cheese evenly on top.
Bake:
Preheat your oven to 350°F (180°C). Bake the dish uncovered for 45-60 minutes, or until the potatoes are fork-tender and the top is golden and bubbly.
Enjoy!
This dish pairs wonderfully with roasted meats, grilled vegetables, or as a standalone indulgence.