We came up with this soup years ago simply by using two things we always have plenty of at Thanksgiving: turkey stock and leftover mashed potatoes. Now we make even this when we don’t have leftover potatoes. This soup is quite delicate and soothing after all the Thanksgiving gluttony.
Put the potatoes in a pot of salted cold water. Bring to a boil over medium-high heat and cook until very soft, 20–30 minutes. Drain off almost all the water, then mash the potatoes with a potato masher until very smooth, adding the milk and butter as you go.
Stir in enough of the hot turkey broth to make a smooth, velvety soup. Season with salt and pepper. Serve the soup hot with a sprinkling of chopped chives.