Heat a skillet over medium heat. Add the ground beef, onion, and garlic. Cook until the beef is browned and the onion is soft. Drain any excess fat.
Transfer the beef mixture to the slow cooker. Add the crushed tomatoes, beef broth, tomato paste, Italian seasoning, salt, and pepper. Stir until everything is well combined.
Cover and cook on low for 6 hours or on high for 3 hours.
About 30 minutes before the end of cooking, stir in the broken lasagna noodles. Replace the lid and continue cooking until the noodles are tender.
Ladle the soup into bowls. Top each serving with a spoonful of ricotta, a sprinkle of mozzarella, and a dusting of Parmesan.