Place the chicken breasts or thighs in the bottom of the slow cooker.
In a medium bowl, whisk together the chicken broth, heavy cream, cream cheese, grated Parmesan cheese, sun-dried tomatoes, minced garlic, Italian seasoning, and red pepper flakes until well combined.
Pour the creamy mixture over the chicken in the slow cooker, ensuring the chicken is fully submerged.
Cover the slow cooker and cook on low for 4-6 hours or on high for 2-3 hours, until the chicken is cooked through and tender.
Once the chicken is fully cooked, remove it from the slow cooker and shred it using two forks. Return the shredded chicken to the slow cooker.
Stir in the penne or short pasta and fresh spinach leaves, mixing well to incorporate everything.
Cover and cook on high for an additional 30 minutes, or until the pasta is al dente and the spinach is wilted.
Before serving, garnish with freshly chopped basil for an added burst of flavor.