Wipe mushrooms with damp cloth. Carefully remove stems and set aside.
In a small, heat-resistant, non-metallic bowl or cup melt 3 tablespoons butter or margarine in Microwave Oven 30 seconds.
Brush mushroom caps with melted butter or margarine and place, hollow-side-up, in a shallow, heat-resistant, non-metallic baking dish. Sprinkle mushroom caps with salt and pepper to taste.
Finely chop reserved mushroom stems.
In a medium-sized, heat-resistant, non-metallic bowl, melt 1/4 cup butter in Microwave Oven 30 seconds. Add onions and chopped mushroom stems; stir to combine.
Heat, uncovered, in Microwave Oven 3 minutes or until onions and mushrooms are tender. Blend in flour until smooth.
Gradually stir in heavy cream until smooth.
Heat, uncovered, in Microwave Oven 2 1/2 minutes or until thickened and smooth. Stir in chopped parsley.
Fill each mushroom cap with some of mushroom mixture.
If desired, sprinkle a little grated Swiss cheese over each stuffed mushroom (refrigerate if not going to serve them at this time).
Heat, uncovered, in a heat-resistant, non-metallic pie pan in Microwave Oven 3 minutes just before serving. (lf refrigerated, heating will take an additional 1 minute.)