Wash mushrooms and remove stems; place on baking sheet and set aside.
Remove sausage from casing. Cook sausage and onion in a frying pan, crumbling sausage with a spatula, until it is cooked through.
Remove from heat. Add cream cheese, green pepper, and pizza sauce and stir well to combine.
Fill each mushroom cap with cream cheese/sausage mixture. Divide filling equally - caps should be completely full and slightly mounded.
Top each mushroom cap with 2-3 tsp. shredded mozzarella cheese.
Bake at 400 degrees for 10-12 minutes.
*Note: mushrooms will be juicy....you may want a plate!