In your slow cooker, combine the tomato pasta sauce, diced tomatoes, minced garlic, salt, Italian seasoning, dried basil, and black pepper.
In a separate bowl, whisk together the cornstarch and heavy cream (or evaporated milk). Stir this mixture into your sauce until fully combined.
Place your chicken breasts gently into the slow cooker, ensuring they are submerged in the sauce.
Cover your slow cooker and set it on low for 3 to 5 hours, until the chicken reaches an internal temperature of 165°F.
Just before serving, stir in freshly chopped basil for an extra burst of flavor.