Chicken, Grape and Walnut Salad

20 oz (3 large) chicken breasts (sautéed and diced)
6 celery stalks (diced)
½ yellow onion (diced)
3 TBS raspberry vinegar
1 tsp tarragon
½ cup mayonnaise
½ cup plain yogurt
½ # red grapes (each cut in ½)
1 ¼ cups walnuts (chopped)
1 tsp kosher salt
½ tsp white pepper

Combine all ingredients well in a medium bowl. Refrigerate several hours to allow flavors to marry and infuse. Serve on choice of bread, roll or on greens. Makes 3 or 4 sizable sandwiches.