Preheat your oven to 375 degrees F. Line a baking sheet with foil and then lightly grease it with non-stick cooking spray or olive oil.
Snap the stems from the mushroom caps and place the caps onto the baking sheet, top side down. Sprinkle the inside of the caps with a bit of salt and pepper. Finely chop the mushroom stems.
Heat the olive oil in a skillet over medium-high heat. Cook the mushroom stems and chopped onion in the oil for 4 minutes. Add the garlic to the pan and cook for an additional minute. Remove from the heat.
In a medium mixing bowl, stir together the softened cream cheese, shredded mozzarella, shredded asiago, parsley, and mushroom mixture until well combined. Fill each mushroom cap with a heaping spoonful of the cream cheese mixture.
In a small bowl, combine the breadcrumbs and parmesan cheese. Dip the cream cheese side of each mushroom into the panko mixture pressing down lightly to coat the tops of the mushrooms with the bread crumbs.
Drizzle the mushrooms with the melted butter and bake for 20-25 minutes or until golden brown.
Remove from the oven, transfer the mushrooms to a platter, and serve warm.
Notes
Don't sweat it if the cheese melts out of the caps a bit. The crispy cheese on the pan makes the mushrooms even better!