Finely chop the mushroom stems, onions and celery and sauté in butter or olive oil until tender. Add sausage and cook until done, breaking it up into small pieces. Drain off excess fat. Add enough bread crumbs to make the mixture hold together.
Stuff mushroom caps, sprinkle shredded cheese over the top and bake for 20 minutes, or until the cheese has melted.
Good to make ahead of time, just add the cheese before baking.