Brown the Italian sausage in a large skillet over medium heat, breaking it up as it cooks. Add the diced onion and cook until softened. Stir in the minced garlic for one minute, then drain any excess fat.
Transfer the sausage mixture to a slow cooker. Add the crushed tomatoes, beef broth, tomato paste, Italian seasoning, salt, and pepper. Stir to combine.
Cover and cook on low for 6 to 7 hours.
About 30 minutes before serving, add the broken lasagna noodles to the slow cooker and stir. Replace the lid and continue cooking until the noodles are tender.
Turn off the heat. Stir in the ricotta, mozzarella, and Parmesan until the cheeses melt and create a creamy texture throughout the soup.