In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Add cheese and mustard; cook and stir until cheese is melted. Cool slightly.
Flatten each biscuit into a 4-in. circle; place four biscuits in each of two greased 15-in. x 10-in. x 1-in. baking pans. Spoon 2 heaping tablespoons of meat mixture onto each.
Using a sharp knife or cookie cutters, cut out jack-o'-lantern faces from remaining biscuit circles; place over meat mixture and pinch edges to seal tightly. Reroll scraps if desired and cut out stems for pumpkins.
Brush with egg. Bake at 350° for 10-15 minutes or until golden brown. Yield: 8 servings.