1. Heat coals or gas grill for direct heat. In a tightly covered container, shake all basting sauce ingredients.
2. Brush mushrooms and onion slices with basting sauce. Cover and grill mushrooms over medium heat 5 minutes. Add onions. Cover and grill about 5 minutes, turning and brushing mushrooms and onions with sauce occasionally, until vegetables are tender.
3. Brush sides of buns with remaining sauce. Fill each bun with lettuce, mushroom, tomato and onion.
*Personal touch - We added a slice of pepperjack cheese to each mushroom while on the grill.