Rub the turkey breast with olive oil, salt, black pepper, garlic powder, onion powder, thyme, and rosemary until evenly coated.
Place the seasoned turkey breast in the slow cooker. Pour the chicken broth around the turkey.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the turkey is cooked through and reaches an internal temperature of 165°F.
Once cooked, remove the turkey from the slow cooker and let it rest for 10 minutes before slicing.
For the gravy, if desired, mix cornstarch with water in a small bowl until smooth. Stir this mixture into the broth in the slow cooker and cook on high for an additional 10 minutes to thicken.
Serve the sliced turkey with the gravy poured over the top.