In a large bowl, gently break up, but do not mash the drained tuna. Add the grapes, walnuts, diced celery, onion and chopped egg. Stir to mix. Add dressing and combine until evenly moistened. Keep chilled in a covered container until ready to serve. Makes 4 servings. (Yield: 3 cups of Tuna Salad.) To make Lemon Tarragon Dressing: Combine all ingredients in a large bowl, then gently combine with tuna mixture.