Preheat oven to 400 F. Grease 9 to 10 inch sqaure baking dish.
Separate mushroom caps and stems; set caps aside. Finely chop stem
pieces. Melt butter over medium heat. Add onion; saute until soft,
about 3 minutes. In bowl, combine sauteed onion, chopped mushroom
stems, bread crumbs, sour cream and hard-cooked egg. In a bowl,
beat egg with fork. Add to onion mixture. Stir in parsley, salt and
pepper until blended. Pack about 1 rounded teaspoon stuffing
mixture into each reserved mushroom cap. Arrange, stuffing-side up,
in greased dish. Place 1 cheese square on each cap. Bake,
uncovered, 20-25 minutes.
