Combine the mashed potatoes, egg, salt, and pepper in a bowl. Mix well. Add enough flour to form a stiff dough. Turn the dough out onto a floured work surface and roll out to about 1/2 to 3/4-inch thick (or as desired). Cut the dough into 2-inch rounds using a biscuit cutter (or other shaped cutter, or form by hand). To cook the dumplings, drop them into boiling soup, stew, or salted water. Cover and simmer for 10 minutes. If cooking in water, drain and serve with your favorite sauce or gravy.