In a large bowl, mix together 1/4 cup of the tomato soup, ground beef, bread crumbs, egg, onion, salt and pepper using your hands. Shape firmly into two loaves.
Heat a large deep skillet over medium-high heat. Brown the meatloaves in the hot skillet, then reduce heat to medium and cover with a lid. Simmer for about 25 minutes, or until meat is cooked through.
Spoon the fat from the skillet, and stir in the remaining soup, water, and mustard. Top with mozzarella cheese, and let simmer uncovered for 10 minutes, stirring sauce occasionally.
