Preheat your oven to 420℉. Line a large rimmed sheet pan with parchment paper.
Wipe clean or peel your mushrooms. Slice them medium thickness.
Sprinkle the cheeses onto your prepped sheet pan evenly spreading out to form a thin layer. Add the mushrooms to the pan so that they are very close together - touching but not overlapping. Sprinkle on salt, onion powder and black pepper to taste. Bake for 15-17 minutes or until the edges of the cheese turn golden brown. Watch it at the end it can burn in seconds.
Remove from oven promptly and optionally lightly brush the tops of the mushrooms only with olive oil. Allow to cool a bit and then break apart. with your hands or a knife. Optionally top with parsley and enjoy!