01 - Season both sides of the chicken breasts with salt and pepper.
02 - Place the seasoned chicken breasts in the bottom of the slow cooker.
03 - In a large bowl, combine crushed tomatoes, diced tomatoes, onion, garlic, tomato paste, oregano, thyme, sugar, red pepper flakes, and olive oil. Stir until well mixed.
04 - Pour the 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 the chicken is tender and reaches an internal temperature of 165°F.
06 - During the last 30 minutes of cooking, stir in the chopped fresh basil and heavy cream if making a creamy sauce.
07 - Taste the sauce and adjust salt, pepper, or red pepper flakes as needed.
08 - Serve hot, garnished with extra fresh basil and Parmesan cheese if desired. Pair with rice, pasta, or crusty bread.