In a large pot or Dutch oven, cook the ground sausage or beef over medium-high heat, breaking it up as it cooks, until no longer pink, about 5-7 minutes. Drain any excess fat.

Add the diced onion and minced garlic to the pot and cook for 2-3 minutes, until the onion is translucent.

Stir in the tomato paste, dried oregano, dried basil, and red pepper flakes (if using). Cook for 1 minute to toast the spices.

Pour in the broth, diced tomatoes, and tomato sauce. Bring the mixture to a simmer and let it cook for 10 minutes.

In a separate saucepan, whisk together the milk, heavy cream, and flour until smooth. Cook over medium heat, stirring constantly, until the mixture thickens, about 5-7 minutes.

Slowly pour the creamy sauce into the simmering soup, stirring constantly to incorporate it. Let the soup simmer for an additional 5-10 minutes, or until it has reached your desired thickness.

Season the soup with salt and black pepper to taste.

Serve the Creamy Lasagna Soup hot, topped with shredded mozzarella cheese and chopped fresh basil.