Squeeze pork sausage out of casings. Clean mushrooms and remove stems. Chop stems finely.
In a large skillet, over medium heat, melt the butter and saute mushroom caps for 2-3 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; saute about minutes, or until the meat loses its pink color. Add the chopped stems and the garlic and sauté 2 minutes.
Remove skillet from heat and add the bread crumbs, sherry, parsley, oregano, sausage-onion mixture and salt; mix well; 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 2-3 minutes or until the cheese is melted.