Cook bacon, mushrooms and onion in a frying pan over medium heat until onion is tender but bacon is not crisp. Remove from heat and combine with cream cheese in a bowl. Trim crust off bread slices and roll slices flat. Spread 2 tbsp cream cheese filling on each slice and roll up, securing with a toothpick. Place rolls seam side down on a parchment-lined baking sheet. Cover and chill in the refrigerator for at least an hour. When ready to bake, brush rolls with melted butter and bake at 375F for 10-15 minutes or until browned. Remove toothpicks and cut rolls in half to serve. Makes 30 appetizers.