Scoop out the pulp of the avocados and cut up coarsely.
Save the shells.
Chop the shrimp coarsely and combine with the avocados.
Shake together vigorously the salt, pepper, garlic powder, mustard, oil, vinegar, and onion.
Pour over the shrimp-avocado mixture and blend well but carefully.
Chill at least 1 hour.
Pour the salad into the avocado shells and serve.