Set your oven to 375F.
Coat a baking pan with cooking spray or use a piece of parchment paper.
Remove the stems from the mushrooms and set aside
Remove the casing from the sausage
Put 1 tbsp butter in a pan.
Add the garlic and onion, sauteing for 4 minutes until they’re fragrant. Set aside.
Crumble the sausage into the pan. Cook for 5 minutes, making sure the sausage doesn’t clump.
Transfer the sausage into a large bowl, adding the garlic mixture and cream cheese. Give it a good stir.
Add the parmesan and Monterey Jack cheese and parsley
Stir again until everything is well integrated.
Put the mixture evenly in the mushroom caps
Melt the last two tbsp of butter. Stir in your breadcrumbs.
Sprinkle the breadcrumbs over the mushrooms.
Put in the oven for about 20 minutes. The breadcrumbs should be golden brown and the mushrooms cooked through. You can do a toothpick test on the mushroom meat to be certain.
Garnish with parsley.