Cut large pieces of steak into roughly 4-inch-wide pieces. Place a single layer of steak in the bottom of a slow-cooker. Peel and slice onion into ¼ -inch-wide rings. Place half of the onion rings over the steak. Sprinkle half of the pepper over the onions. Layer the rest of the steak over the rings and top with the remaining onion slices and pepper (see note).
In a small bowl, combine soup, gravy mix and lukewarm water and mix well. Pour gravy mixture over steak and cover slow-cooker. Cook on high heat for 6 hours or low heat for 10 hours. Remove steak to a large platter and spoon gravy over steak. Serve immediately.
Makes 4 servings.