* Hamburger buns should be the small ones or use 6 slices of bread. ~- Heat oven to 500 degrees F. Spread cut sides of hamburgers buns or one side of each bread slice with mustard. Mix meat, onion, salt and pepper. Spread mixture over the mustard, being careful to bring it to the edges of the buns. Place meat sides up on an ungreased baking sheet. Bake until desired doneness is reached, about 5 minutes. Note: If you like, you can have these burgers ready and waiting in the freezer for last-minute cooking. After spreading the meat mixture over the buns, wrap each securely in heavy-duty or double thickness of regular aluminum foil and label; freeze no longer than 2 months. To serve, unwrap desired number of hamburgers and bake about 10 minutes.