Boil, drain, rinse and peel the potatoes. Cut the onion into very small cubes, cut the jacket potatoes into slices about 1 cm thick
Melt the butter in a saucepan and sauté the onion cubes until translucent. Now add the potato slices and fill up with part of the milk until the potatoes are half covered. Bring to a simmer over medium heat, stirring gently. Add the parsley and stir until it has a creamy consistency. It thickens by itself due to the potato starch. Then season with salt, pepper and nutmeg to taste.
Here in North Frisia, it is served either with meatballs, fish or with warm sausages.