In a large skillet, cook ground beef and Italian sausage over medium-high heat until browned, about 8 minutes. Transfer to slow cooker.
Add onion and garlic to the slow cooker alongside meat.
Pour in beef broth, marinara, diced tomatoes, tomato paste, and red pepper. Stir to combine.
Cover and cook on LOW for 6–8 hours (or HIGH for 3–4 hours).
About 30 minutes before serving, stir in broken lasagna noodles and spinach; cover and cook until pasta is al dente.
Stir in parmesan, then ladle into bowls, topping each with mozzarella and dollops of ricotta.
Notes
You can substitute ground turkey or omit sausage; adjust cook time if using gluten‑free noodles. Store in fridge up to 4 days; reheat and add extra broth if needed.