Toast the walnuts in a dry frying pan until fragrant, let them cool, and coarsely chop them. Toasting keeps them crunchy in the creamy dressing; raw straight from the bag, they will become soft and soggy within an hour.
For the dressing, whisk the Greek yogurt with the mayonnaise, mustard, one tablespoon of lemon juice, salt, and pepper. The classic Waldorf is drenched in mayonnaise alone; this version, made mostly with yogurt, is fresher and better suited for lunch.
Slice the celery into thin crescents. Halve the grapes. Cut the unpeeled apple into matchsticks and toss immediately with a squeeze of lemon juice to prevent browning.
Dice the smoked chicken breast and toss it with the celery, apple, and grapes into the dressing. Let the salad sit in the refrigerator for ten minutes to allow the flavors to meld.
Tear the little gem lettuce into leaves and arrange them as cups on a plate. Fill them with the salad and garnish with the walnuts, plenty of chives, and a crack of black pepper.