In a large pot or Dutch oven, brown the Italian sausage over medium-high heat. Break it up with a spoon. Drain off any excess grease.
Add the chopped onion to the pot and cook until softened, about 5 minutes. Stir in the minced garlic and cook for another minute until fragrant.
Pour in the crushed tomatoes, chicken broth, dried oregano, dried basil, and red pepper flakes (if using). Stir to combine and bring the mixture to a simmer.
Add the uncooked elbow macaroni to the simmering soup. Cook according to the pasta package directions, stirring occasionally, until the pasta is al dente.
Stir in the ricotta cheese and grated Parmesan cheese until well combined. Season with salt and pepper to taste.
Ladle the soup into bowls. Top each serving with shredded mozzarella cheese and chopped fresh parsley.
Notes
This lasagna soup is delicious served with crusty bread for dipping.