Cut crusts off of bread and roll out thinly with rolling pin. Saute onion and mushrooms in half the butter until onions are soft. Mash the onion-mushroom mixture into the cream cheese and add salt & pepper.
Spread about 1 Tbsp of mixture on each piece of bread & roll up. Cut each roll in half. Melt remaining 3 Tbsp butter and brush rolls with butter. Bake at 350º for 15 minutes. (makes 52 hors d'oeuvres)