Place a rack in center of oven and preheat to 425 degrees. Brush or wipe mushrooms and then slice off stems even with the bottom of cap.
Place mushrooms into a 13“ x 9” baking dish with the stems facing up. Group mushrooms as closely together as possible in a single layer.
Drizzle melted butter and sprinkle garlic salt and Parmesan evenly over mushrooms. Bake 20–22 minutes or until Parmesan is melted and edges begin to get slightly crispy.
Remove from oven, sprinkle with fresh chopped parsley and season to taste with a little additional garlic salt, if necessary. Serve mushrooms nice and hot as a side dish or appetizer.