Saute onion in oil. Add beef and let it brown while breaking it up. Add ketchup, mustard, salt, and pepper.

Preheat oven to 350°F (180°C).

Remove pie crust from pan. Lay flat on a baking sheet and arrange filling in center, leaving a one-inch (two-centimeter) border. Fold the edge of pie dough in, covering the filling just slightly.

Brush dough with egg wash and bake at for 45 minutes or until nicely browned.