Whisk together all sauce ingredients in a medium sauce pan. Bring to a boil, reduce to simmer and cook for 10 to 15 minutes. Remove from heat and allow to cool to room temperature. Reserve 1/2 cup of sauce and pour remaining sauce in a large sealable bag. Add chicken breasts to bag. Seal and refrigerate for 30 minutes. Preheat grill to medium heat and arrange four 12x12-inch pieces of Reynolds Wrap® Non-Stick Foil on the grill. Use tongs to transfer one chicken breast to each foil square. Discard marinade. Brush each chicken breast with some of the remaining BBQ sauce. Top with sliced mushrooms and onions. Fold foil around each chicken breast to enclose the chicken and veggies. Allow to cook over medium heat for 10 to 12 minutes or until chicken is tender and juices run clear or meat thermometer reads 170 degrees F. Unwrap foil packets and place two slices of provolone cheese over the top of each chicken breast so that the cheese is lying on top of the mushrooms and onions. Allow to cook uncovered 2 to 3 minutes longer or until cheese is melted. Garnish with fresh parsley or cilantro and serve.
Lock in delicious flavor by grilling chicken and veggies together in a Reynolds Wrap® Non-Stick Foil packet.