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.
Notes
You can make this salad up to two days in advance and store it its the fridge, but don’t add the toasted walnuts until you’re ready to serve it.