Combine mushroom stems, onion and garlic in a food processor until finely chopped.  Heat oil in skillet and add mushroom mixture.  Saute 4-5 minutes.  Return mixture to processor and add the next 7 ingredients.  Process until smooth, add additional butter if needed.  Set aside to cool to room temp.

Preheat oven to 400 degrees. 

Heat 3 tbsp butter in skillet and quickly saute mushroom caps on high for 1-2 minutes.  Place on lightly greased baking sheet and spoon small amount of stuffing in each .

Bake 5-10 minutes or until heated throughout