In a small microwave-safe bowl, sprinkle gelatin over cold water; let stand for 1 minute. Microwave, uncovered, on high for 20 seconds. Stir; let stand for 1 minute or until gelatin is completely dissolved.
In a large saucepan, combine the soup, cream cheese, mayonnaise and gelatin. Cook and stir over medium heat for 5-7 minutes or until smooth. Remove from the heat; stir in the crab, celery and onions.
Transfer to a 5-cup ring mold coated with cooking spray. Cover and refrigerate for 4 hours or until set. Unmold onto a serving platter; serve with crackers.