1. In a large bowl, gently break up, but do not mash the drained tuna.
2. Add the grapes, walnuts, diced celery, onion and chopped egg. Stir to mix.
3. Add the dressing and combine until evenly moistened.
4. Keep chilled in a covered container until ready to serve.

To make Lemon Tarragon Dressing:
Combine all ingredients in a large bowl. Store in the refrigerator in a tightly covered container for no longer than 1 week.