Rinse off turkey breast and pat dry. Place turkey in pan, breaking breast bone if necessary to flatten it so that grill cover will close completely. Preheat grill on high. Place turkey on grill; close cover. Reduce heat to low. Combine remaining ingredients in a small saucepan. Cook over medium heat just until sauce is smooth, whisking frequently. Set aside. Liberally brush glaze on turkey after first hour of cooking. Brush on additional glaze about every half hour until turkey is done, about 2-3 hours. A meat thermometer inserted into the thickest part of the breast, without touching bone, will register 170º when turkey is fully cooked. Let sit 15 minutes before carving.