When fresh tarragon isn't available, substitute 1 1/2 tsp. dried crushed tarragon.
Arrange pecans in a single layer on a baking sheet.
Bake at 350° for 5 to 7 minutes or until lightly toasted. Cool pecans on a wire rack 15 minutes or until completely cool.
Whisk together mayonnaise and next 5 ingredients in a large bowl; stir in pecans, chicken, celery, and onion just until blended. Stir in grape halves, if desired. Garnish, if desired.
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