Step 1: Arrange the Chicken - Place the chicken breasts in the bottom of your slow cooker in as even a layer as you can manage. If one breast is noticeably thicker than the others, put it toward the hotter outside edge of the cooker if your model tends to cook more strongly there. There is no need to brown the chicken first. That is part of why I like this recipe on a busy day. You can go straight from prep to cooking without dirtying another pan.
Step 2: Build the Savory Sauce - Add the onion powder, dried thyme, garlic powder, salt, black pepper, minced garlic, tomato paste, chicken broth, grated Parmesan, and chopped sun dried tomatoes around and over the chicken. Give the liquid a gentle stir so the tomato paste begins to dissolve and the seasonings are distributed through the broth. Do not worry if the Parmesan does not completely disappear into the liquid yet. As everything heats, those flavors will settle together and season both the chicken and the sauce.
Step 3: Slow Cook the Chicken - Cover the slow cooker and cook on High for about 2 to 3 hours, with roughly 3 hours being a good target for average sized chicken breasts. Cooking time can vary depending on the thickness of the chicken and the way your slow cooker runs, so start checking on the earlier side if your chicken breasts are thin. The chicken is ready when the thickest portion reaches 165°F. I try not to leave chicken breast cooking much longer than necessary because that is how a tender dinner can turn dry.
Step 4: Mix the Cream Thickener - About 1 hour before you expect the chicken to be finished, give the sauce a gentle stir. In a small bowl, whisk the cornstarch into the heavy cream until the mixture looks completely smooth. Take an extra few seconds here. Cornstarch can form stubborn little lumps if it hits hot liquid while still dry, and mixing it into cold cream first is the easiest way to avoid that problem.
Step 5: Thicken the Creamy Parmesan Sauce - Pour the cream and cornstarch mixture into the slow cooker and stir gently to combine it with the cooking liquid. Try to work around the chicken rather than breaking it apart. Cover the cooker again and continue cooking for about 1 more hour, or until the sauce is noticeably thicker and creamy. By this point, it should look less like broth and more like a sauce you want to spoon generously over potatoes or pasta.
Step 6: Finish with Fresh Basil - Once the chicken is cooked through and the sauce has thickened, taste a small spoonful of sauce and adjust the salt or pepper if needed. Sprinkle fresh basil over the chicken just before serving. I like to tear the leaves with my fingers rather than fussing with a knife because this is usually the point when everyone is standing nearby with plates in hand. Spoon plenty of sauce over each piece of chicken and make sure every serving gets some of those sun dried tomatoes.