01Preheat the oven to 375 degrees F. Prepare a 13x9 baking dish with cooking spray.
02Clean mushrooms with a damp paper towel brushing off dirt. Carefully break stems from mushrooms. Save the stems and chop stems extremely fine.
03In a small skillet, melt the butter over medium heat and sauté the chopped mushroom stems for 3 minutes until browned and softened.
04Add the grated onion and minced garlic to skillet and sauté for another minute.
05Add in the panko crumbs and cook one more minute until the panko becomes brown. Be careful not to burn the garlic or onion.
06Into the skillet add the softened cream cheese, Parmesan cheese, cheddar cheese, salt and pepper and mix well.
07Use a spoon to pack the cavity of each mushroom with a generous amount of cheese filling.
08Place about 1/8 cup of grated Parmesan cheese on a small plate and press each cheese-stuffed mushroom top (cheese side down) into grated Parmesan cheese to create a crust. Then place the mushrooms in the baking dish with the cheese side up.
09Bake for 20-25 minutes, until the stuffed mushroom tops are golden brown. Serve warm.