01 - Season chicken breasts thoroughly with salt and pepper on both sides.
02 - Place seasoned chicken in the bottom of the slow cooker.
03 - Combine crushed tomatoes, diced tomatoes, onion, garlic, tomato paste, oregano, thyme, sugar, red pepper flakes, and olive oil in a large bowl. Mix until well incorporated.
04 - Pour tomato mixture evenly over the chicken in the slow cooker.
05 - Cover and cook on LOW for 6 hours or HIGH for 4 hours, until chicken reaches internal temperature of 165°F.
06 - During the last 30 minutes of cooking, stir in chopped fresh basil and heavy cream if making a creamy version.
07 - Taste sauce and adjust salt, pepper, or herbs as needed.
08 - Serve hot, garnished with additional basil and Parmesan cheese. Accompany with rice, pasta, or crusty bread.