Remove stems from mushroom caps. Chop stems into small pieces and put into a large bowl. Add remaining ingredients and mix together with your hands so that the mixture sticks together. If mixture is too dry, add another beaten egg.
Place mushroom caps in heavy baking pan and fill each cap with a mound of mushroom mixture. Use leftover mixture to cover mushrooms already stuffed.
Cover pan and bake at 180ºC or about 30 minutes or until cheese is bubbling. Serve immediately.