1Arrange shrimp in single layer in shallow glass or plastic container.
2Heat soy sauce to boiling in 1-quart saucepan; add gingerroot. Reduce heat; simmer 5 minutes or until most of the liquid is absorbed. Stir in vinegar, sugar, sake and salt. Pour over shrimp. Cover with plastic wrap and refrigerate at least 2 hours but no longer than 12 hours.
3Remove shrimp from marinade with slotted spoon; arrange on serving plate. Sprinkle with onions.