To a slow cooker, add the onion, garlic, tomato paste, diced tomatoes, chicken broth, chicken breasts, Italian seasoning, and red pepper flakes — cover and cook for 4 hours on high or 8 hours on low.
In the last 30 minutes, add the pasta, 1 ¼ cups Parmesan cheese, and ¼ cup Mozzarella cheese to the slow cooker. Stir to combine ingredients, cover, and cook for 25-30 minutes, until pasta is ready.
Serve:
To serve, fill bowls and sprinkle with remaining Parmesan and Mozzarella cheeses.