In a large pot or Dutch oven over medium heat, cook the Italian sausage, 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 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour in the chicken broth and diced tomatoes with their juices. Stir to combine.
Add the dried Italian seasoning, crushed red pepper flakes, salt, and black pepper. Bring the soup to a boil.
Once boiling, 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, about 4-5 minutes, until the ravioli float to the surface and are tender.
Stir in the chopped baby spinach and cook for an additional 1-2 minutes until wilted.
Remove the pot from heat and stir in the grated Parmesan cheese.
Ladle the soup into bowls and garnish with additional Parmesan cheese if desired. Serve hot.