Prepare the slow cooker by coating it with a light coating of cooking spray.
Add the chicken breasts directly into the slow cooker and season with salt, pepper, Italian seasoning, and crushed red pepper flakes.
Add the chopped onions, garlic, and red bell pepper over the chicken, then pour in the crushed tomatoes and chicken broth.
Cover and cook on Low for 6–7 hours or on High for 3–4 hours, until the chicken is tender and fully cooked.
Remove the chicken from the slow cooker, shred or chop it into bite-sized pieces, and return it to the soup.
Add the dry ditalini pasta, stir well, and cook on High for 20–25 minutes or until the pasta is tender.
Stir in the mozzarella and Parmesan cheeses until melted and creamy.
Ladle into bowls and top with croutons and fresh basil before serving.