Assemble in Slow Cooker: Add chicken, diced tomatoes, tomato sauce, chicken broth, garlic, salt, pepper, basil, oregano, and red pepper flakes to the slow cooker. Stir to combine.
Cook Low & Slow: Cover and cook on low for 6–7 hours or high for 3–4 hours, until chicken is tender.
Add Cream & Cheese: Stir in heavy cream, Parmesan cheese, and butter. If a thicker sauce is desired, mix in the cornstarch slurry and cook for another 15 minutes.
Serve & Enjoy: Garnish with fresh basil and extra Parmesan. Serve over pasta, rice, or with crusty bread.