1In small bowl, place shrimp and onions. In tightly covered container, shake oil, salt, tarragon and vinegar; pour over shrimp. Sprinkle with pepper; toss until shrimp are well coated. Cover; refrigerate at least 2 hours.
2Drain shrimp; reserve marinade to pour over salads if desired. Toss grapes with shrimp mixture. Serve on lettuce.