In a bowl, combine the first four ingredients. Crumble beef over mixture and mix well. Shape into six patties. Place in a greased 11x7-in. baking dish. Spread mustard over each patty. Top with onion and cheese. Pour soup over all. Bake, uncovered, at 350° for 35 minutes or until the meat is no longer pink. Serve on buns.
You won't use an entire can of tomato soup in Easy Oven Patties. So freeze the little bit that's left over and add it to the pot next time you make vegetable soup or spaghetti sauce.