1In large bowl, mix beef, egg, 1/4 cup of the Bisquick mix and the pepper. With wet hands, shape beef mixture into 6 oval patties, about 1/2 inch thick. Lightly coat patties with remaining 1/4 cup Bisquick mix.
2In 12-inch nonstick skillet, heat oil over medium heat. Arrange patties in single layer in skillet. Add onion slices around and on top of patties. Cook 12 to 16 minutes, turning patties once and stirring onions occasionally, until meat thermometer inserted in center of patties reads 160°F.
3If necessary, spoon fat from skillet and discard. Add mushrooms and gravy to skillet. Turn patties to coat with gravy. Heat to boiling.