Prepare the Base: Add tomato pasta sauce, canned diced tomatoes, minced garlic, salt, Italian seasoning, dried basil, and black pepper to the slow cooker. Stir well.
Whisk the Cream: Mix heavy cream and corn starch until smooth, then add to the slow cooker and combine with the tomato base.
Add Chicken: Place chicken breasts in the slow cooker, ensuring they are coated and slightly submerged in the sauce.
Cook: Set slow cooker to low (3-4 hours) or high (2-2.5 hours) until chicken reaches an internal temperature of 165°F.
Finish with Fresh Basil: Stir in chopped fresh basil before serving for added freshness and flavor.