Squeeze pork sausage out of casings. Chop mushroom stems finely. In a large skillet, over medium heat, melt the butter and sauté mushroom caps for 2 to 3 minutes or until slightly golden. Remove from pan with a slotted spoon and drain, upside down, on paper towels.
Reduce heat slightly. Add sausage and onions to the skillet and sauté for about 5 minutes, or until the meat loses its pink colour. Remove sausage mixture with a slotted spoon. If necessary, drain excess fat from the pan. Add the chopped stems and the garlic and sauté for 2 minutes.
Remove skillet from heat and add the breadcrumbs, sherry, parsley, oregano, sausage-onion mixture and salt. Mix well and let cool slightly. Stir in the mozzarella cheese. Spoon stuffing mixture into mushroom caps and refrigerate.
When ready to serve, place stuffed mushrooms on a lightly greased baking sheet. Place under a broiler for 2 to 3 minutes or until the cheese is melted.