Trim away any fat and mince the meat. Mix the meat and the well-mashed potato very thoroughly. Season with salt, pepper and Worcestershire sauce. Separate the egg, adding the yolk to the mixture and reserving the white for coating the patties.
Turn the mixture out on to a clean working surface and divide equally into four or six portions. With lightly floured hands, shape each portion into a round patty. Lightly mix the egg white with a fork. Place the crumbs on a plate or square of greaseproof paper. Dip each patty first in the egg white to coat all sides, then roll in the toasted breadcrumbs. Pat the coating on firmly and set the patties aside until ready to fry.
Add to the hot fat, melted, in a frying pan. Fry for 3–5 minutes or until golden brown on both sides. Drain and serve.