In a large skillet over medium heat, brown the ground beef (or sausage) until fully cooked. Drain any excess fat.
Add the diced onion and minced garlic to the skillet with the meat. Sauté for about 3-4 minutes until the onion becomes translucent.
Transfer the meat mixture to the slow cooker.
Stir in the crushed tomatoes, tomato sauce, broth, Italian seasoning, salt, and black pepper. Mix well.
Add the broken lasagna noodles to the slow cooker, pushing them down so they are submerged in the liquid.
Cover and cook on low for 6 hours.
After 6 hours, stir in the ricotta cheese, half of the mozzarella, and half of the Parmesan cheese. Mix gently until combined.
Let it heat through for an additional 15-20 minutes.
Before serving, top with the remaining mozzarella and Parmesan cheeses. Allow them to melt slightly in the soup.
Serve hot, garnished with fresh basil leaves if desired.