In a mixing bowl, combine ground beef, egg, onion, crackers, milk, mustard, salt and pepper. Shape into six oval patties.
Fry in skillet over medium heat for 10-12 minutes or until cooked through, turn once. Remove patties and keep in warm oven.
Prepared gravy - add gravy to skillet to heat
Homemade gravy - stir flour into pan drippings stirring with wire whisk, continue stirring add water and bouillon cube, until dissolved. Add salt and pepper to taste. Serve gravy over patties along with potatoes or noodles.