Remove mushroom stems. Chop enough stems to measure 1/2 cup. Cook half of the mushroom caps in 3 tablespoons margarine over medium heat 5 minutes. Drain. Repeat with remaining mushroom caps and margarine.
Combine cream cheese and blue cheese, mixing until well blended.Stir in chopped stems and onions. Fill mushroom caps. Place on cookie sheet. Broil until golden brown.