Preheat the oven to 375°F (190°C). Grease a large (2 quart) baking dish with butter.

Peel the potatoes and cut them into thin slices.

Melt the butter in a small pot over medium heat. Add the flour and whisk constantly for 1-2 minutes until it turns into a thick paste.

Gradually pour in the milk, whisking constantly to prevent lumps. Add the onion, garlic, salt, and pepper and continue to whisk until the mixture thickens.

Remove the saucepan from heat and add half the grated Gruyere cheese, whisking until melted and combined.

Arrange a layer of sliced potatoes in the prepared baking dish. Pour a layer of the cheese sauce over the potatoes, making sure to cover them completely. Repeat the process, layering the potatoes and cheese sauce until all the potatoes and sauce are used up.

Sprinkle the remaining Gruyere cheese on top.

Bake the dish for 45-50 minutes, until the potatoes are tender and the cheese is golden brown and bubbly on top.

Let the dish cool for a few minutes before serving.