Mix butter and thyme; brush over turkey. Cover and grill turkey, breast side up, over drip pan or over unheated side of gas grill and 5 to 6 inches from medium heat 2 hours 30 minutes to 3 hours 30 minutes or until thermometer reads 165° and drumsticks move easily when lifted or twisted. If using charcoal grill, add about 15 briquettes every hour. Let turkey stand 15 minutes. Remove onion and lemons before carving.