Chop mushroom stems. Melt butter in med. size saucepan; brush outside of mushroom caps. Cook chopped stems with onion in remaining butter 'til tender. Add cream cheese and mustard, stirring until smooth. Stir in crab, water chestnuts and pimiento; fill mushroom caps. Sprinkle with cheese. Bake in shallow pan at 400 degrees for 10-15 mins. or 'til hot.