In a large skillet over high heat, brown the Italian sausage for 4 minutes. Break apart the sausage as it browns.
Add onion and garlic, then cook for another 1–2 minutes.
Remove from heat and drain the extra grease. Transfer the cooked sausage and onion to a slow cooker.
To the slow cooker, add the tomatoes, chicken broth, tomato paste, Italian seasoning, pepper flakes, salt, and pepper. Stir to combine well.
Cook on high for 3 hours, then add the noodles and stir to incorporate.
Set to low heat, then cook for another 30 minutes.
To serve, scoop into bowls and garnish with a tablespoon of ricotta cheese, chopped parsley, and grated Parmesan cheese.
