POUR 1-1/4 qt. (5 cups) dressing over chicken in 8-gal. plastic container; turn to coat both sides of each breast. (Or, spoon 1 Tbsp. dressing over chicken in small plastic container for trial recipe.) Pour 1-1/4 qt. (5 cups) of the remaining dressing over mushrooms in separate plastic container; turn to coat both sides of each. (Or brush mushroom with 1 Tbsp. of the remaining dressing for trial recipe. Place in small separate plastic container.) Refrigerate chicken and mushrooms 20 min. to marinate.
REMOVE chicken from marinade; discard marinade. Grill chicken on medium-high heat 6 min. or until done (165ºF), turning occasionally to grill-mark both sides. Remove from grill.
REMOVE mushrooms from marinade; discard marinade. Grill 2 min. or until tender, turning occasionally to grill-mark both sides. Remove from grill.
FOR each sandwich (2 servings): Heat panini grill; spray with cooking spray.
BUTTERFLY 1 chicken breast; slice 1 mushroom. Spread 2 bread slices evenly with 1 Tbsp. of the remaining dressing. Top 1 bread slice with 1 cheese slice, sliced mushrooms, chicken, 2 bacon slices and 1 tsp. each basil and Parmesan. Cover with remaining bread slice.
GRILL 3 to 5 min. or until golden brown. Cut in half.