01 - Season the chicken breasts evenly with salt and freshly ground black pepper on both sides, ensuring full coverage for maximum flavor penetration.
02 - Arrange the seasoned chicken breasts in a single layer at the bottom of the slow cooker, avoiding overlap for even cooking.
03 - In a large mixing bowl, combine crushed tomatoes, drained diced tomatoes, finely chopped onion, minced garlic, tomato paste, dried oregano, dried thyme, sugar, red pepper flakes if using, and olive oil. Whisk thoroughly until all ingredients are fully incorporated.
04 - Pour the tomato mixture evenly over the chicken breasts, ensuring each piece is well-coated with the sauce.
05 - Cover and cook on LOW setting for 6 hours or HIGH setting for 4 hours until chicken reaches internal temperature of 165°F and is tender throughout.
06 - During the final 30 minutes of cooking, stir in the chopped fresh basil. If preparing the creamy version, add heavy cream or half-and-half at this stage and stir gently to combine.
07 - Taste the sauce and adjust seasoning with additional salt or pepper if needed. Serve immediately, garnished with extra fresh basil and Parmesan cheese if desired. Accompany with rice, pasta, polenta, or crusty bread.