I got this recipe from a neighbor over thirty years ago, and I've used it often to take to parties and get-togethers. The only problem is keeping others from eating them before I can get them into the car.
Preheat oven to 350 degrees. Break out the stem from the mushrooms and place on cookie sheet.
In frying pan, cook sausage over medium heat until brown. Then drain grease. Add breadcrumbs and mozzarella cheese and stir to mix. Stuff into mushroom caps. Bake 10-15 minutes until cheese starts to brown on top and the mushrooms are sizzling.