Oven temperature set to 375 degrees Fahrenheit (190 degrees C). It is necessary to butter a 9x13-inch baking pan.
Peel and thinly slice the potatoes.
Over medium heat, place a large skillet to melt the butter in.
Sauté the onion until softened after adding it.
Stir the flour to make a roux.
Pour the milk in gradually and continuously whisk to avoid lumps.
Simmer the mixture for two to three minutes, or until it thickens.
Add the cheese, then taste and adjust the salt and pepper.
In the lined baking dish, arrange the potato slices in layers.
Over the top, pour the sauce.
Bake the dish for 30 minutes with the foil covering it.
Bake for an additional 30 minutes but after removing the foil or until the potatoes are soft and their top becomes golden brown.
Before serving, allow to cool for a while. Enjoy!