Remove mushroom stems; chop enough stems to measure 1/3 cup. Cook and stir chopped stems and onion in 1 Tbsp. of the butter in medim skillet until tender. Stir in bread crumbs.
Add to cream cheese in medium bowl; mix until well blended. Gently stir in crabmeat.
Melt remaining 2 Tbsp. butter; brush evenly onto mushroom caps. Fill caps with cream cheese mixture; place, filled sides up, in shallow baking dish.