1. Pat the boneless, skinless turkey breast dry with paper towels. In a small bowl, combine the dried thyme, garlic powder, onion powder, black pepper, and salt. Rub this seasoning mixture all over the turkey breast, ensuring it is evenly coated.

2. Heat the olive oil in a large skillet over medium-high heat. Once hot, carefully place the seasoned turkey breast in the skillet and sear for 2-3 minutes per side, until lightly golden brown. This step is optional but recommended for added flavor.

3. Transfer the seared (or unseared) turkey breast to your slow cooker. In a separate bowl, whisk together the apple cider, apple cider vinegar, brown sugar, and Dijon mustard. Pour this mixture over the turkey breast in the slow cooker, ensuring the turkey is partially submerged in the liquid.

4. Cover the slow cooker with its lid. Cook on the LOW setting for 6-8 hours, or on the HIGH setting for 3-4 hours. The turkey is done when an instant-read thermometer inserted into the thickest part of the breast registers 165 F (74 C). The turkey should be fork-tender.

5. Once the turkey is cooked, carefully remove it from the slow cooker and place it on a cutting board. Tent it loosely with foil to rest while you prepare the glaze. Skim any excess fat from the cooking liquid in the slow cooker. In a small bowl, whisk together the cornstarch and cold water to create a smooth slurry.

6. Pour the cornstarch slurry into the cooking liquid in the slow cooker. Stir well. Turn the slow cooker to the HIGH setting (or transfer the liquid to a saucepan on the stovetop over medium-high heat). Cook, stirring frequently, until the glaze thickens to your desired consistency, usually about 10-15 minutes. It should be thick enough to coat the back of a spoon.

7. Slice the rested turkey breast against the grain into thick pieces. Arrange the slices on a serving platter and generously drizzle with the warm apple cider glaze. Serve immediately.