When cooking with leftovers, it is not possible to give exact measurements, it depends on how many potatoes you got. The water/vinegar ratio is 2 part water to 1 part vinegar.
Slice your onions and fry them until they are translucent. Slice your potatoes and add them to the onions in the pot.
Try to slice your potatoes into even slices.
Add water and vinegar. Add sugar and salt to your taste.
Boil without lid, until the potatoes have thickened the water/vinegar. 5-10 minutes, depending on what kind of potatoes you are using. Taste again and add sugar and or salt if it needs it.
Add a tablespoon or two of cream to round the taste a bit, you don't have to. Serve with almost any kind of fried meat or fish.