Preheat oven to 350°F. Wash the peppers and cut around the stems to remove the tops. Remove the seeds and membranes and rinse the insides. Melt the butter in a small skillet over medium heat. Add the onion and sauté until tender and translucent. In a large bowl, combine the ground beef, sautéed onion, cooked rice, egg, salt, and black pepper. Mix gently until combined. Spoon the beef mixture evenly into the peppers. Arrange the stuffed peppers upright and close together in a roasting pan or baking dish. In a large bowl, combine the condensed tomato soup with the amount of water directed on the cans and stir until smooth. Pour the tomato soup over the peppers. Cover and bake for 2 hours, or until the peppers are tender and the filling reaches an internal temperature of 160°F. The tomato soup will cook down and thicken into a gravy as the peppers bake. Serve with mashed potatoes and vegetables, if desired, spooning the tomato gravy over the peppers and potatoes.