01 - Season chicken breasts thoroughly with salt and black pepper on both sides.
02 - Arrange seasoned chicken breasts in an even layer at the bottom of the slow cooker.
03 - In a large mixing bowl, combine crushed tomatoes, diced tomatoes, chopped onion, minced garlic, tomato paste, oregano, thyme, sugar, red pepper flakes, and olive oil. Mix thoroughly until all ingredients are well incorporated.
04 - Pour the tomato mixture evenly 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. If preparing the creamy version, add heavy cream or half-and-half at this time and mix well.
07 - Taste sauce and adjust seasoning as needed. Serve hot over pasta, rice, or with crusty bread. Garnish with additional fresh basil and Parmesan cheese if desired.