Pop off the stems to the mushrooms and chop them finely. Set aside. In a medium skillet brown the sausage and drain. Set aside to cool. In the same skillet add the olive oil, onion, garlic, and chopped mushroom stems. Sauté for about 3 minutes or until tender. Set aside to cool.

In a medium sized bowl add cream cheese and egg yolk. Stir together with parmesan cheese and parsley. Add cooled sausage and onion mixture and combine until incorporated.

Stuff each mushroom with the mixture making a small mound on the top. Place on a baking sheet and bake mushrooms at 350 degrees for 25 minutes until they are slightly brown on the top.