Remove stems from mushrooms and set caps aside; discard stems or save for another use. In a small bowl, combine the cream cheese, blue cheese, onions, garlic and 1/4 cup bacon bits. Stuff into mushroom caps, a scant tablespoonful in each.
Place in a greased 15-in. x 10-in. x 1-in. baking pan. Sprinkle with remaining bacon bits. Bake, uncovered, at 375° for 18-22 minutes or until mushrooms are tender.