Preheat the grill for high heat.Advertisement
Melt the butter in a skillet over medium heat. Place the onion slices in the skillet, sprinkle with garlic powder, and saute until onions are translucent. Remove from heat, and set aside.
Lightly oil the grill grate. Place the chicken breast halves on the grill, and sprinkle with wine, rosemary, and thyme. Cook for about 8 minutes. Turn, and continue cooking 8 minutes, or until juices run clear. Place a slice of cheese on each breast half, and top with onions. Cook until cheese starts to melt. Serve on toasted English muffins.