Preheat broiler. Remove stems from mushrooms and chop finely. Reserve caps. Saute the stems and onion in 1/4 cup butter or margarine until tender.
Stir in the bread crumbs, pistachios, parsley, marjoram, and salt, mixing well. Remove from heat.
Place the mushroom caps, stem-side up, on a baking sheet. Fill the centers with the stuffing and sprinkle with about 3 tablespoons melted butter.
Broil about 6 inches from heat source for 5 minutes or until browned and heated through.