Remove stems from mushrooms, set aside. Saute mushroom caps in 1 tsp oil for 3 minutes. Remove caps and set on a tray.
Mushrooms have a lot of water, so you have to dry them slightly with paper towel. Chop mushroom stems and saute them in remaining oil and juice. Add minced onion and cook until mixture is soft and golden.
Remove mixture from heat. Stir in cream cheese and Parmesan. Fill each mushroom with small baking sheet and broil until filling bubbles. Cool slightly before serving.