Using a meat tenderizer, pound chicken as thin as possible. Season front and back of chicken to taste with garlic salt and pepper. Place seasoned chicken into a bowl, cover with raspberry vinegar dressing and seal with plastic wrap. Place bowl in refrigerator for least 4 hours. Turn chicken several times to make sure it is well coated with the dressing. Remove chicken from marinade. Evenly sprinkle a thin layer of crumbled Bleu Cheese over each breast. Pepper the chicken. Roll up each chicken breast, wrap with one or two pieces of bacon and secure with toothpick.
On a large sheet of heavy duty aluminum foil, spread rings of onion in center and sprinkle with mushrooms. Drizzle with raspberry vinegar dressing. Set wrapped chicken on top of onions and mushrooms. Fold sides of foil so that it creates a pouch and the juices do not escape.
Set grids at medium or high. Preheat grill to medium heat. Place foil with chicken on grill for 35 to 45 minutes (until the internal temperature of the chicken is 180°F). To check temperature, unroll top of the foil (watch out for steam!) and test chicken with a meat thermometer.
