Heat a large pot over medium heat. Add the Italian sausage and cook until browned, breaking it into crumbles. Drain excess grease.
Add diced onion and minced garlic to the pot. Sauté until soft and fragrant, about 3–4 minutes.
Stir in tomato paste, crushed tomatoes, and chicken or beef broth. Season with Italian seasoning, salt, and pepper. Bring to a boil.
Reduce heat to low and let the soup simmer for 15–20 minutes to allow the flavors to meld together.
Add broken lasagna noodles to the pot and cook until tender, about 10–12 minutes. Stir occasionally to prevent sticking.
Stir in ricotta cheese for a creamy texture.
Serve the soup hot, topped with shredded mozzarella, Parmesan, and fresh spinach (if desired). Enjoy with crusty bread!