In a medium bowl, combine the mashed potatoes and chopped turkey; season with salt and pepper. (If you like, add Parmesan cheese, garlic, herbs or spices.) In a shallow dish, beat the eggs with a fork. Put the breadcrumbs into another shallow dish.
Shape the mashed potato mixture into small balls, patties or logs. Roll in the beaten egg to coat, and then in breadcrumbs to coat. (Use one hand to dip them in the egg, and keep the other dry to help coat them with crumbs.) Place them on a baking sheet as you coat them.
Heat an inch or two of oil in a heavy pot set over medium-high heat until it’s hot, but not smoking. A few breadcrumbs should sizzle when dropped in, and if you have a thermometer it should read about 350?F.
Cook the croquettes in batches, without crowding the pot, turning them as they turn golden, about 3-5 minutes. Transfer to a paper towel lined plate with a slotted spoon, and sprinkle with salt while they’re still warm. Serve immediately, with cranberry sauce, green sauce or garlic aioli, if you like. Serves about 4.