Chop underdone roast beef fine and mix with one-third as much smoothly mashed potato. Season to taste with pepper, salt, and mustard. Knead lightly, and when the ingredients are well incorporated, work in a beaten egg to bind the mixture. Set aside to cool and stiffen. When ready to cook them, roll them in beaten egg and cracker-dust, leave them in a cold place again for an hour or so, and fry in boiling dripping. Serve dry.