In a large skillet over medium-high heat, brown ground beef with diced onion and garlic for 6-8 minutes until fully cooked. Drain excess fat.
Transfer cooked beef to a 6-quart slow cooker. Add crushed tomatoes, tomato paste, beef broth, Italian seasoning, red pepper flakes, salt, and pepper.
Cover and cook on LOW for 6 to 7 hours (or HIGH for 3 to 4 hours).
30 minutes before serving, stir broken lasagna noodles into the slow cooker broth. Cover and cook on HIGH for 30 minutes until noodles are al dente.
Ladle soup into bowls. Top generously with seasoned ricotta, shredded mozzarella, grated Parmesan, and fresh basil. Serve immediately!