Wipe off mushrooms with a damp rag. Pop out stems, reserving both parts.

Chop mushroom stems finely and set aside.

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 he at. 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 parsley if you're feeling fancy.