Pieczarki nadziewane (stuffed mushrooms)

20    Mushrooms
      2 tb butter/margarine
      1 sm Onions; minced
      3 tb Bread crumbs, dry
      1 tb Sour cream
      1    Eggs; hard-cooked, minced
      1    Eggs
      1 tb parsley; chopped
    1/4 ts Salt
    1/8 ts Pepper, black
     20 sl Cheese, gouda; 1" square
           -or swiss cheese
 
    Preheat oven to 400F. Grease 9 to 10 inch sqaure baking dish. Seperate
  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.