Cook the potatoes until they're done.
Peel the potatoes and cut them into slices about 5mm thick.
Peel the onions, halve them, and slice them very thinly into half-rings.
Melt the butter and set it aside.
In a large bowl (where the potatoes will be mixed later), squeeze half a lemon, and mix the vinegar, paprika powder, honey, and mustard into a dressing.