In a large skillet over medium heat, cook the Italian sausage until browned, breaking it apart with a wooden spoon as it cooks. Drain excess fat if necessary.
Add the chopped onion and minced garlic to the skillet and sauté until the onion is translucent, about 5 minutes.
Transfer the sausage mixture to the slow cooker.
Stir in the crushed tomatoes, broth, Italian seasoning, salt, and black pepper. Mix well.
Cover and cook on low for 4 hours.
About 30 minutes before serving, break the uncooked lasagna noodles and add them to the slow cooker, stirring gently to combine.
In a separate bowl, mix the ricotta cheese with half of the mozzarella and half of the Parmesan. Stir until well combined.
After the noodles are tender, spoon some of the soup into bowls and add a generous dollop of the ricotta mixture.
Top each bowl with the remaining mozzarella and Parmesan. Let it melt slightly before serving.
Garnish with fresh basil or parsley if desired. Serve hot.