1. Rinse mushrooms and pat dry. Remove and chop stems.
2. Mix chopped stems with crab, cheese, onions, and Worcestershire.
3. Place mushrooms, cup side up, in a buttered 9- by 13-inch pan. Spoon crab mixture equally onto mushrooms, then press lightly to fit within caps.
4. Bake in a 350° oven until cheese melts and browns lightly, 20 to 25 minutes. Serve hot.