Mix the egg(s), mashed potatoes, spices, and flour until the mixture is firm enough to handle gently. Pat them into a patty or roll them into a cylinder, roll in bread crumbs, then lay on a plate covered with wax paper. Refrigerate an hour if you have time.
Heat a thin layer of oil and fry the croquettes, about 5 minutes on each side. They should be browned and crispy on the outside, soft on the inside.