Wash mushrooms in water and dry and remove stems from caps. (it's okay to wash mushrooms they will not absorb the water)
Finely chop mushroom stems.
Brown and crumble sausage. Set aside on a plate to cool.
Add onions and garlic to the same skillet; cook for 2 minutes over medium low heat. Pour in wine to deglaze pan, allow liquid to evaporate. Add in chopped mushroom stems, stir to cook for 2 minutes. Add salt and pepper to taste. Set mixture aside on a plate to cool.
In a bowl, combine cream cheese and egg yolk. Stir together with Parmesan cheese.
Add cooled sausage and cooled mushroom stems. Stir mixture together and refrigerate for a short time to firm up.
Smear mixture into the cavity of each mushroom, creating a sizable mound over the top.
Bake at 350 degrees for 20 to 25 minutes, or until golden brown. Allow to cool at least ten minutes before serving; the stuffed mushrooms taste better when not piping hot. Garnish with minced flat leaf parsley.