Crumble ground beef in a large skillet over medium heat. Cook and stir for 5 minutes. Drain.
Place beef in the bottom of a slow cooker. Add bell pepper, onion, diced tomatoes, crushed tomatoes, beef broth, mushrooms, and Italian seasoning. Cook on low 4 hours.
Add broken lasagna noodles. Cook on Low until noodles are tender, about 45 minutes more.
Ladle soup into serving bowls. Divide mozzarella cheese between the bowls. Top each bowl with a dollop of ricotta cheese.
