Heat the olive oil in a large skillet over medium-high heat. Add the Italian sausage and ground beef, breaking up the meat with a wooden spoon. Cook, stirring occasionally, until browned, about 6–8 minutes. Drain excess grease if needed and transfer the meat to the slow cooker.

In the same skillet, add the chopped onion and cook until softened, about 4–5 minutes. Stir in the garlic and tomato paste, cooking for 2 minutes to deepen the flavor. Add the red pepper flakes if using.

Transfer the onion mixture to the slow cooker. Add the crushed tomatoes, chicken broth, Italian seasoning, oregano, basil, salt, and sugar. Stir to combine.

Cover and cook on low for 4–5 hours or high for 2–3 hours. About 30 minutes before serving, stir in the broken lasagna noodles. If the soup is too thick, add a splash of water or broth. Continue cooking until the pasta is al dente.

In a small bowl, combine the ricotta, mozzarella, and Parmesan cheeses. When the soup is ready, stir in the fresh spinach (if using) until wilted. Ladle the soup into bowls and top with a dollop of the cheese mixture and additional shredded mozzarella, Parmesan, and fresh basil.