Be prepared to fry the potatoes twice: once to cook them so they fluff inside and again to get them crispy on the outside. When I use an electric fryer, I always follow the manufacturer's instructions. To make good fries in a pot on the stove, a deep-fat thermometer is essential. It ensures that the oil is at the proper temperature for deep-frying and lets you check that the oil isn't overheating, a potentially dangerous situation. A mandoline is a very useful slicing tool for cutting the potatoes (and other vegetables) quickly and to a uniform size. Both the deep-fat thermometer (also called a candy thermometer) and the mandoline are available at most cookware stores. The third essential is a pot that is large and tall enough to contain the oil without overflowing when the potatoes are slipped in.
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.