PREHEAT oven to 350°F. 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.
BAKE 18 to 20 minutes or until heated through.