Put some oil on a baking sheet or a container suitable for baking. Any amount of potatoes, washed but unpeeled, halve with a knife or cut into thick slices. Spread the oil on the baking container with one of the halved potatoes and neatly distribute the remaining potatoes on top.
Bake the potatoes at approx. 230 ° C top / bottom heat on a medium level in the oven until they are golden-brown and bubble.
As soon as the potatoes start to bubble, switch off the oven and distribute a few slices of sliced cheese evenly on the potatoes.
Wait until the cheese has melted on the potatoes and refine everything with ketchup and mayonnaise.