Boil the potatoes for about 15 minutes. They should still have a bit of bite to them.
Let the potatoes cool slightly, peel them, and then cut them into slices.
Peel the onions and cut them into thin slices.
Put onions and butter in a pot and melt the butter.
Add sugar, vinegar, and water to the pot with onions and butter, and bring the mixture to a boil. Let it simmer for 15 minutes.
Add the boiled potatoes to the vinegar/onion mixture.
Let it simmer until it has the right consistency. It can be a bit porridge-like.
Season the warm potato salad with salt and pepper.
For example, serve the potato salad with boiled or grilled sausages with mustard and ketchup.