In a small bowl, combine the diced apples and lemon juice. Toss well. In a large bowl, dump in the mayonnaise and mustard and whisk until uniform. Add the cubed chicken, acidulated apples, grapes, celery, and tarragon to the large bowl. Use a spatula to mix everything together. Taste for seasoning and add salt and pepper to taste. Serve on a bed of lettuce. Top with toasted walnuts right before serving.