Remove stems from mushrooms (save for use in soups and stews).
Heat butter or margarine in large skillet.
Add mushroom caps and saute for 5 minutes, turning occasionally.
Remove mushrooms to baking pan, cap-side up; set aside.
Saute onion or scallions in the same skillet.
Add remaining ingredients and mix well.
Fill sauteed mushroom caps with shrimp mixture.
Bake at 450° for 6 to 8 minutes.