Melt the butter in a frypan over medium heat. Place the onion slices in the frypan, sprinkle with garlic powder and saute until onions are translucent. Remove from heat and set aside.
Lightly oil the barbecue grate. Place the chicken breast halves on the barbecue 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.