01 - Season chicken breasts thoroughly with salt and freshly ground black pepper on both sides.
02 - Place seasoned chicken breasts in a single layer at the bottom of the slow cooker.
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, and olive oil. Mix thoroughly until all ingredients are well incorporated.
04 - Evenly distribute the tomato mixture over the chicken breasts in the slow cooker.
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 chopped fresh basil. For a creamy version, add heavy cream or half-and-half at this stage and mix well.
07 - Taste sauce and adjust seasoning as needed. Serve hot, garnished with additional fresh basil and grated Parmesan cheese if desired. Accompany with rice, pasta, or crusty bread.