Prep the mushrooms: clean the mushrooms under running water with your hands, and pat them dry with paper towels. With a paring knife, remove the stems (but save them for another dish!) and spread out the mushroom caps on a parchment-lined baking sheet stem side up.
Preheat the oven to 375°F/190°C.
In a skillet, cook the sausage breaking it apart with a wooden spoon until it’s no longer pink. If there’s a lot of grease in the pan, drain it reserving about 1 tablespoon.
Add the onion and cook for 3 minutes or until the onion is translucent.
Add the garlic, and cook for 30 seconds. Remove from heat.
Place the sausage in a bowl, add cream cheese and parmesan. Stir to combine.
Divide the sausage mixture between the mushrooms.
In a small bowl, combine the breadcrumbs with butter, and sprinkle over the mushrooms.
Bake in the oven for 20 minutes or until the mushrooms are cooked through and the stuffing is browned and crusty.
Garnish with parsley and serve on a platter.