If you love mushrooms, you gotta' love these......they are quick, & easy to fix.....can be fixed ahead, covered with plastic wrap and stored in fridge until time to place under broiler. I fixed ahead, set out of fridge about 30 minutes before broiling.
Remove mushroom stems; chop enough stems to measure 1/2 cup. Cook half of 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, onions and crumbled bacon; fill mushroom caps. Place on cookie sheet; broil until golden brown.