Wash and peel mushrooms. Remove stalks and discard. Brush mushrooms with melted butter. Fry onions in remaining butter. When tender, mix onion and butter with breadcrumbs, sausagemeat, herbs and seasonings. Divide among the mushrooms. Place mushrooms in a shallow ovenproof dish, pour 4 tablespoons of water into the bottom of the dish, and bake for 15-20 minutes in a moderate oven.