Preheat oven to 400 degrees F (200 degrees C). Grease a 9-inch pie plate with cooking spray.
In a medium bowl, combine tuna fish, onion, egg, bread crumbs, mayonnaise, mustard, salt, and pepper. Mix well. Pour into the prepared pie plate. Drizzle with the melted butter or margarine.
Bake for 20 minutes, or until golden brown. Serve with lemon wedges, if desired. Enjoy!