• Preheat oven to 375 F. Grease a large sheet pan with nonstick cooking spray, or place a square of parchment paper or a silicone baking mat. Set aside.
• Wash the mushrooms and dry well. Remove the mushroom stems and set aside on a cutting board. Using a spoon, scrape out the mushroom gills and discard - these are not worth saving as they will create too much moisture. Set hollowed out mushrooms spaced apart on the prepared cookie sheet. Set aside.
• Chop the mushroom stems, and set aside.

For the Filling:

• Melt 1 ½ tablespoon of butter in a frying pan over medium heat. Add in the onion and minced garlic, stirring frequently for 3 to 4 minutes, until softened. Scrape the mixture out into a large mixing bowl and set aside.
• Add the chopped mushroom stems to the pan and cook until just softened, about 3 to 4 minutes. Drain any juices, then scrape out the mushrooms into the bowl with the onions and garlic.
• Add in the sausage, crumbling it into smaller bits as it browns. Cook all the way through, about 5 to 6 minutes. Add the sausage to the bowl with the onions, garlic, and mushroom stems.
• To the large bowl with the sausage mixture, add the cream cheese, Parmesan cheese, Monterey Jack cheese, and 4 tablespoons of fresh chopped parsley. Stir until completely combined.
• Spoon the filling into each mushroom cap, packing it down with the back of a spoon to fit in as much as you can. If there's any extra filling, you can bake it in a small dish by itself for less time and eat it as a dip - a snack for the chef.

For the Breadcrumb Topping:

• Add the remaining 3 tablespoons of butter to a small bowl, then microwave 30 seconds or so until melted. Stir in the breadcrumbs until moist.
• Sprinkle the breadcrumbs over the stuffed mushrooms. Alternatively, you can dip each mushroom into the breadcrumbs for them to adhere more evenly.
• Bake for 20 minutes, or until the breadcrumbs are golden brown, the cheese is melted, and the mushrooms are tender. Sprinkle with additional fresh chopped parsley. Then, serve. If a lot of juices released from your mushrooms while baking, you may want to re-plate before adding the parsley and serving.
• Enjoy!