In a large pot or Dutch oven over medium heat, add the Italian sausage and cook, breaking it up with a spoon, until browned and cooked through, about 5-7 minutes.
Add the diced onion to the pot and cook until softened, about 3-4 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour in the chicken broth, diced tomatoes with their juice, and tomato sauce. Stir to combine.
Add the dried Italian seasoning, crushed red pepper flakes, salt, and black pepper. Stir well.
Bring the soup to a boil, then reduce heat to medium-low and simmer for 10 minutes to allow flavors to meld.
Add the cheese ravioli to the pot and cook according to package instructions, usually about 3-5 minutes, until the ravioli float to the top and are tender.
Stir in the fresh baby spinach and cook for 1-2 minutes until wilted.
Remove the pot from heat and stir in 1/4 cup grated Parmesan cheese.
Ladle the soup into bowls and garnish with additional Parmesan cheese if desired. Serve hot.