Prep the chicken: If using rotisserie chicken, remove skin and shred or dice the meat into bite-size pieces.
If cooking from scratch, poach or roast chicken breasts until just done, then cool and chop.
Make the dressing: In a large bowl, whisk together mayonnaise, Greek yogurt, Dijon, lemon juice, salt, and pepper until smooth. Taste and adjust acidity and seasoning.
Add the crunch: Stir in celery and red onion. If using grapes or apple, fold them in gently so they stay intact.
Combine: Add the chicken to the bowl and toss to coat evenly.
Sprinkle in herbs and nuts, then stir again. The mixture should be creamy but not soupy.
Chill (optional but better): Cover and refrigerate for at least 20–30 minutes to let the flavors meld.
Assemble the sandwiches: Toast bread if you like. Layer with lettuce and tomato, then pile on the chicken salad.
Top with the second slice of bread and press gently.
Serve: Slice in half and serve with chips, a pickle, or a simple green salad.