Take a large pan, pour in the butter and briefly add a little heat and let it melt, turn off the stove. The butter must not get hot. Add the flour and stir with the whisk, so that a soft pulp is formed without lumps. Slowly add the milk, stir and bring to the boil. Season to taste.
Now add the potato slices, onions and 50 grams of cheese. Mix everything together, without heat, and pour into a gratin dish. Spread the rest of the cheese on top.
Bake in the preheated oven at approx. 180 ° for 40 minutes. Use as an accompaniment to meat or sausages.
Tip:
also add bacon, ham, leek or vegetables of your choice.