Meanwhile, place the pecans and walnuts on a separate sheet pan and toast in the oven for 7 to 8 minutes until golden. Set aside to cool.
05For the dressing, mix together the mayonnaise, sour cream, 2 teaspoons salt and 1/2 teaspoon pepper. Fold in half the chopped tarragon leaves.06Place the diced chicken in a bowl, add the pecans, walnuts and grapes. Pour the dressing over the chicken and toss well. Sprinkle the remaining chopped tarragon leaves on top, and serve on a bed of lettuce leaves.