Boil 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 to make a dressing.