1. Brown pork sausage and crumble well.
2. Drain sausage from skillet leaving a thin coating of oil behind and sauté chopped onions until slightly wilted.
3. Return sausage to pan with onions and add cream cheese.
4. Over low heat stir sausage, onions, and cream cheese until well blended.
5. Remove stems from mushrooms and stuff each mushroom cap with the mixture.
6. Top with parmesan if desired.
7. Bake at 350°F for 20-25 minutes until heated through and the mushrooms are beginning to leak their water.
8. Note: Sauté mushroom stems and use any leftover filling in an omelette.