In a small mixing bowl, whisk together mayonnaise, yogurt, poppy seeds, and tarragon to make dressing; set aside.
In a medium mixing bowl, combine chicken, grapes, celery, and walnuts. Pour 1/2 cup of dressing over mixture and stir to combine; set aside.
Put spring lettuce mix in large mixing bowl with 1/4 cup dressing. Use tongs to toss.
Arrange 1 1/2 cups of spring mix lettuce on each plate. Top with 1 cup of tarragon chicken. Serve with remaining dressing on the side.