Combine the diced tomatoes, marinara sauce, minced garlic, salt, Italian seasoning, dried basil, and pepper into your slow cooker. Stir well to ensure everything is evenly mixed together.
In a small bowl, whisk together the heavy whipping cream and cornstarch until smooth. Add this mixture to the slow cooker and stir it into the tomato mixture.
Place the boneless skinless chicken breasts into the slow cooker, making sure they are fully submerged in the sauce. Press them down gently so they soak up all those delicious flavors.
Cover your slow cooker and set it to cook on low for about 3 to 4 hours. Use a meat thermometer to check that the internal temperature of the chicken reaches 165°F.
Once cooked, stir in freshly chopped basil and taste your sauce; adjust seasonings as needed based on your preferences. Serve hot with grated Parmesan cheese on top for added flavor.