Heat up the grill.
Coat both sides of the mushroom caps with cooking spray.
Place mushroom caps, 'gill' side down, on grill rack also coated with cooking spray.
Grill for 7 minutes (not 5 as the recipe states) on this side and then turn the mushroom caps over.
With the 'gill' side now facing up, place a slice of Provolone on top of each mushroom cap.
Generously sprinkle the shredded Mozzarella cheese on top of the provolone cheese.
Generously sprinkle the Parmigiano cheese on top of the Mozzarella cheese.
Grill for 10 minutes (not 5 as the recipe states) or until cheese is golden and melted.