Drain beans in a colander and transfer to a bowl. Gently stir lemon vinaigrette into warm beans. Beans may be prepared up to this point 1 day ahead. Cool, uncovered, before placing covered in refrigerator.
Gently mix celery, scallions, olives, parsley, garlic, and tarragon. Stir in additional more vinaigrette if too dry. Add salt and pepper to taste.
Can be made a day ahead. Store dressing and bean mixture separtely. Add dressing right before serving.
LEMON VINAIGRETTE:
1/3 cup fresh lemon juice (about 3 lemons)
1/2 - 2/3 cup extra-virgin olive oil (to taste)
In a bowl combine lemon juice with salt and pepper to taste. Whisk in oil in slow stream. .