Peel and slice the carrots and parsnips. Place them at the bottom of the slow cooker, and add the peeled smashed garlic cloves. In a small bowl, mix together the poultry seasoning, salt, and black pepper and set to the side.
Rinse the turkey breast, pat dry, and place in the slow cooker. Add the 4 tablespoons of butter underneath the skin of the turkey breast. Then, sprinkle the seasoning both under the skin and on top for turkey breast. Put the bundle of thyme inside the breast cavity. Place the turkey breast skin-side down and pour in 1/2 cup water.
Cook on low for 6 hours or until internal temperature reaches 165°F. Remove the cooked turkey from the slow cooker and place on a baking sheet.
Preheat the broiler to 500°F. While the oven preheats, drain the drippings from the slow cooker into a saucepan. Heat the drippings on medium heat. In a small bowl, mix together the cornstarch and 1 tablespoon water until it’s well combined. Using a whisk, mix in the cornstarch mixture into the turkey drippings. Whisk until the drippings become thick like gravy. Remove from the heat once the gravy is at your desired consistency.
Place the turkey under the broiler for 3-5 minutes or until the skin becomes golden in color. Slice the turkey and serve with carrots, parsnips, and gravy.