2 cups bread crumbs
1 large egg
2 tablespoons water
2 cups mashed potatoes
2 cups stuffing
2 cups chopped cooked turkey
4 to 6 tablespoons olive oil, as needed
Warm turkey gravy
In a small sauté pan over moderate heat, toast the bread crumbs until golden and crunchy, 3 to 4 minutes. Transfer to a small bowl and let cool.
In a small bowl, beat together the egg and water.
In a large bowl, combine the mashed potatoes, stuffing, and turkey. Using your hands or a spoon, shape the mixture into 2-inch-long, oblong-shaped croquettes (you should have about 24 croquettes) and transfer to a large rimmed baking sheet.
Working with 1 croquette at a time, dredge them in the egg-water mixture, letting the excess drip off, then dredge them in the bread crumbs, and return them to the rimmed baking sheet.
In a large skillet, over moderately high heat, warm 2 tablespoons of the oil. Working in batches, pan-fry the croquettes, turning them occasionally, until crispy on all sides, 5 to 6 minutes total. Repeat with the remaining croquettes, wiping the pan clean and adding 2 tablespoons of the oil to the pan between batches. DO AHEAD: The croquettes can be prepared ahead and frozen, well wrapped in foil, for up to 3 months. To reheat: Bake the frozen croquettes on a large baking sheet in a 475°F oven until golden brown, about 10 minutes. Remove the croquettes from the oven for about 5 minutes to let the interiors thaw, then return them to the oven to finish baking for another 2 minutes.