Brown the ground beef in a skillet over medium heat. Add the diced onion and minced garlic and cook until the onion softens.
Transfer the beef mixture to the slow cooker. Stir in the crushed tomatoes, beef broth, Italian seasoning, salt, and black pepper.
Cover and cook on low for 6 hours.
About 30 minutes before serving, add the broken lasagna noodles to the slow cooker. Stir gently, cover, and continue cooking until the noodles are tender.
Ladle the soup into bowls and top each serving with a generous spoonful of ricotta cheese.