• Preheat the oven to 350°F.
• Spray a baking dish with cooking spray (use whatever size dish you need to fit all the mushrooms; I used an 11x7-inch dish).

Prepare the Mushrooms

• Gently remove the stems from the mushrooms (either give the stem a gentle push to one side or twist it gently until it pops off).
• Place the mushroom caps, cavity-side up, in the baking dish.
• Finely chop the mushroom stems; set aside.

Make the Filling

• In a medium skillet (I use a 10-inch skillet), melt the butter over medium heat.
• Add the chopped mushroom stems, green onions and garlic; sauté, stirring frequently, until softened and the watery liquid released from the mushroom stems has evaporated, about 5 minutes.
• Turn off the heat and add the panko breadcrumbs; stir until well-combined.
• Add the mozzarella and parmesan cheeses; stir until well-combined.
• Taste the filling and season with salt and/or pepper as needed.

Stuff the Mushrooms & Bake

• Spoon the filling into the mushroom caps.

  When you start stuffing these, it's going to seem like there is too much filling, but make sure you use it all. I first go around and stuff each cavity, gently pressing the filling down inside. Then I go back around again and top the mushrooms with the rest of the filling. The mushrooms should be heaping full when you are done.
• Bake for 30 minutes or until the mushrooms are tender but still slightly firm and the tops are golden brown.

  Be careful not to overcook these, as the mushrooms can becometoo soft. If, after 30 minutes, the mushrooms are done but the tops are not quite golden brown, I would put them under the broiler for a minute or two to brown rather than continue baking them.
• Serve immediately. If desired, sprinkle with chopped fresh parsley.