When I make mashed potatoes, I always make more than needed so I can have leftovers for the next nights meal. My potato soup recipe is very similar to making mashed potatoes.. Peel and cube potatoes, place in cold water and put on a high simmer until soft. Drain. While they are cooking, I chop a medium sized sweet onion and saute it in 3 T butter until soft. Add the amount of 2% milk you wish, add onion/butter mixture and a cup of your leftover mashed potatoes. Stir gently until well mixed, simmer until milk is warmed. Add salt and pepper to your liking and enjoy! We have this many times during the winter, some like to add cheese to their soup.