In a small bowl, combine tomato sauce and Italian seasoning; reserve 1/3 cup and set aside covered in refrigerator.
In a large mixing bowl, combine remaining tomato sauce mixture with egg, onion, bell pepper, breadcrumbs, garlic and salt and pepper; blend well, then add ground beef and blend well again. together.
Take 2 strips of aluminum foil, each about 15 inches long, and form an "x"; place in slow cooker.
On a cutting board, form meat mixture into a round and place on top of the foil strips, being careful to NOT let the meatloaf touch the sides of the slow cooker.
Cover and cook on low heat setting for 8 to 10 hours or on high setting for 4 to 5 hours.
During last 10 minutes of cooking time, pour reserved tomato sauce on top of meatloaf and sprinkle with Mozzarella cheese. Cover and cook for the remaining 10 minutes.
Using the foil strips, slowly and carefully pull the meatloaf from the slow cooker onto a serving plate; gently pull foil strips from under meatloaf. Allow meatloaf to rest for about 10 minutes before slicing.