Preheat the oven to 375°F. Coat a sheet pan with cooking spray and arrange the mushrooms on the pan.
Melt 1 tablespoon of butter in a pan over medium heat. Add the onion and garlic and cook for 3-4 minutes or until softened. Remove the onion mixture from the pan.
Add the sausage to the pan and cook for 5-6 minutes, using a spatula to break up the meat into smaller pieces.
Place the sausage, onion mixture, cream cheese, parmesan cheese, Monterey jack cheese and 3 tablespoons of parsley in a bowl. Stir to combine.
Spoon the sausage mixture evenly into the mushroom caps.
Melt the remaining 2 tablespoons of butter in the microwave and stir in the panko breadcrumbs to coat.
Sprinkle the panko over the mushrooms.
Bake for 20 minutes or until tops are browned and mushrooms are cooked through. Sprinkle with parsley, then serve.