Preheat the oven to 375 degrees.
Heat one tablespoon of butter in a large cast iron skillet over medium heat.
Add the onion and cook, stirring occasionally, for about 5-6 minutes, or until softened.
Add the minced garlic and crushed red pepper flakes and cook, stirring constantly, for 1 minute.
Remove the onion from the skillet into a large bowl; set aside.
Return the skillet to the stove over medium high heat. Add the sausage to the hot skillet and cook, stirring often and making sure to break up the meat into crumbles, until just cooked through, about 4-5 minutes.
Pour the sausage crumbles into the bowl with the onions; set aside to cool.
Wipe out the skillet with a towel if needed. Coat the skillet with cooking spray.
Add the softened cream cheese, Monterey Jack cheese, parmesan cheese, and fresh parsley to the bowl with the cooled sausage and onions then season with sea salt and freshly cracked pepper, to taste; mix until well combined.
Spoon the mixture evenly into each mushroom then place into the skillet.
Melt the remaining 2 tablespoons of butter then mix with the panko crumbs.
Sprinkle the mixture evenly over the mushrooms.
Place into the oven and bake for 20-25 minutes, or until bubbling and golden brown.
Sprinkle with freshly chopped parsley. Serve immediately. Enjoy.