Wash the mushrooms and remove the stems

Chop the stems finely and set aside

Melt two tablespoons of butter in a large skillet and gently saute the mushroom caps for two to three minutes or until they are slightly golden, but remove before they are noticeably shrunken

Remove with a slotted spoon and drain on a paper towel

Add sausage and onions to the skillet and saute for about five minutes, until the meat loses its pink color and the onions are crisp-tender

Remove with a slotted spoon

Add the remaining two tablespoons of butter to the skillet along with the chopped mushroom stems and saute another two minutes

Remove from heat and add the bread crumbs, sherry, oregano, parsley, garlic, sausage and onion mixture, salt and pepper, and mix well

Add the mozzarella to the mixture and stir

Place an equal amount of the stuffing mixture in each cap

Place the caps on a greased cookie sheet and put in a preheated broiler for two or three minutes or until the cheese bubbles, or refrigerate until needed.