Boil the jacket potatoes. Shortly before the end of the cooking time, slowly fry the onion cubes in butter until brown (not black!). Put the buttermilk in a pouring vessel and lightly salt. Add the brown butter and onion mixture to the buttermilk and stir well.
Peel the boiled potatoes and cut into slices in soup plates. Pour the buttermilk over them.
It`s a quick, inexpensive dish that we particularly enjoy in summer.