Purchase steamed shrimp at the supermarket. Stuff mushrooms ahead, but bake just before serving.
Remove stems from mushrooms; chop stems.
Spray mushroom caps with butter-flavored cooking spray.
Stir together chopped mushrooms, shrimp, and next 4 ingredients. Spoon evenly into mushroom caps; sprinkle evenly with cheese, if desired. Place on a lightly greased rack in a broiler pan.
Bake at 375° for 20 minutes.