Brown the Italian sausage in a skillet over medium-high heat until no longer pink, then drain excess grease.
Place the cooked sausage into a 6-quart slow cooker.
Add onion, garlic, marinara sauce, diced tomatoes, tomato paste, chicken broth, oregano, basil, salt, and pepper to the slow cooker and stir well.
Cover and cook on high for 4 hours or low for 7-8 hours.
Stir in the broken lasagna noodles during the final 30 minutes of cooking time or until noodles are al dente.
Combine the ricotta, mozzarella, and Parmesan cheese in a small mixing bowl.
Ladle the soup into bowls and serve with a dollop of the cheese mixture and a garnish of fresh parsley.