Clean the mushrooms with a damp paper towel and pull out the stems. Finely chop 1/4 cup of the mushroom stems and set aside.
In a large skillet, brown and crumble the sausage until cooked through. Transfer to a plate to cool.
To the same skillet, add the onion, garlic and 1/4 cup of reserved mushroom stems to the same skillet and cook 4-5 minutes on medium low heat. Season with salt and pepper to taste and remove from heat.
In a large bowl, combine the cream cheese and parmesan cheese. Add the cooled sausage and mushroom stems and stir to combine.
Preheat oven to 350o F. Use a small spoon to scoop the sausage mixture into the cavity of each mushroom. Place the mushrooms on a baking sheet and bake for 20-25 minutes until the tops of the mushrooms just start to brown.