01 - Lightly coat the inside of the slow cooker with olive oil to prevent sticking.
02 - Place the chicken breasts in an even layer across the bottom of the slow cooker.
03 - In a mixing bowl, combine the diced tomatoes with juices, chopped onion, minced garlic, salt, black pepper, dried oregano, dried basil, and crushed red pepper flakes if using. Stir to blend evenly.
04 - Pour the tomato sauce mixture over the chicken breasts, ensuring they are well coated. Cover and cook on low for 6 to 8 hours or on high for 4 hours, until the chicken is tender and reaches an internal temperature of 165°F.
05 - For a richer, creamier sauce, stir in the heavy cream during the last 10 minutes of cooking and allow it to heat through.
06 - Transfer the chicken breasts to serving plates and spoon the tomato basil sauce over the top. Garnish with freshly torn basil leaves and serve immediately.