In a skillet over medium heat, cook the ground beef and onion until the beef is browned and the onion is soft. Drain excess fat.
Add garlic and cook for another minute until fragrant.
Transfer the beef mixture to a slow cooker. Add beef broth, crushed tomatoes, diced tomatoes, tomato paste, basil, oregano, salt, black pepper, and red pepper flakes.
Cover and cook on low for 6-7 hours or high for 3-4 hours.
Thirty minutes before serving, add the broken lasagna noodles and stir.
Cover and continue cooking until the noodles are tender.
Ladle soup into bowls and top each serving with ricotta, mozzarella, and Parmesan cheese.
Garnish with fresh basil and serve hot.