In a bowl, mix THOROUGHLY 1/4 cup soup, beef, bread crumbs, egg and onion. Shape FIRMLY into 6 patties. In a skillet over medium-high heat, cook patties, a few at a time, until browned on both sides. Spoon off the fat. Stir in the remaining soup and mushrooms; return patties to skillet. Reduce heat to low. Cover; simmer 20 minutes, or until done ... turning patties occasionally. Wis/Gramma, 08/26/95.