Remove stems from mushrooms and gently wipe mushrooms clean. Bring a large pot of water to a boil. Drop mushrooms in. Cook 1 minute, then drain in colander and let cool. Cook onion, celery and green pepper in butter until soft. Tear bread into small pieces. Combine bread, onion mixture, crabmeat, mayonnaise and season with salt and pepper and a dash of nutmeg. Dry mushrooms and stuff. Bake on buttered pan at 350° for 20 minutes.