Heat a large skillet over medium-high heat. Add the ground beef and cook for approximately 5–7 minutes, breaking it apart with a wooden spoon, until it is no longer pink.
Add the diced onion and cook for 3–4 minutes, until it begins to soften.
Stir in the minced garlic and cook for approximately 30 seconds, until fragrant.
Drain any excess grease from the skillet. Transfer the browned beef mixture to the crockpot.
Add the marinara sauce, crushed tomatoes, diced tomatoes, tomato paste, beef broth, Italian seasoning, dried basil, dried oregano, salt, black pepper, and red pepper flakes.
Stir the soup ingredients until well combined.
Cover the crockpot and cook on LOW for approximately 4–5 hours, or until the soup is hot and the onions are tender. Use your tested cooking time because slow cookers can vary.
Break the lasagna noodles into smaller pieces. Add them to the crockpot during the final 25–35 minutes of cooking.
Stir the noodles into the tomato broth. Cover and continue cooking until the noodles are tender but still slightly firm.
While the noodles cook, prepare the ricotta topping. In a small bowl, combine the ricotta cheese, mozzarella, Parmesan, Italian seasoning, and black pepper.
Stir the ricotta mixture until smooth. Add the milk or cream if you want a softer, creamier consistency.
Taste the soup and adjust the salt, pepper, Italian seasoning, or red pepper flakes as needed.
Ladle the hot lasagna soup into bowls.
Top each serving with a spoonful of the ricotta mixture.
Garnish with fresh basil, parsley, extra mozzarella, Parmesan, or red pepper flakes.
Serve immediately with garlic bread, crusty Italian bread, breadsticks, or a simple green salad.