Mix all marinade ingredients except the cornstarch in a mixing bowl. Stir with a whisk to combine well. Set aside 1/2 cup of the marinade for the glaze.
Cut tenderloin in half lengthwise (i.e. butterfly the tenderloin). Place in a container with a tight-fitting lid. Pour marinade over tenderloin and marinate for at least 2 hours or overnight.
When ready to grill, preheat grill to hot. Place meat directly on the grill and cook for about 5 minutes per side, or until a meat thermometer reads 165 F when inserted into the turkey tenderloin.
Meanwhile, heat remaining marinade in a small sauce pan. Slowly whisk in cornstarch and cook, stirring all the while, until sauce thickens slightly to coat the back of a spoon. This makes a perfect glaze for the tenderloin.
Remove meat from grill and drizzle glaze over meat.
