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-inch circle; place four biscuits greased 15x10x1-inch 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. Re-roll scrapes if desired and cut out stems for pumpkins. Brush with egg. Bake at 350° for 10 to 15 minutes or until golden brown. Yields 8 servings. Nancy Tune, Salem, Mo.