Prep the chicken. Dice or shred the cooked chicken into small, bite-size pieces. Aim for even pieces so the salad mixes well.
Make the dressing. In a large bowl, stir together mayonnaise, Greek yogurt (if using), Dijon, lemon juice, salt, and pepper. Taste and adjust creaminess and acidity.
Add the crunch and color. Fold in celery, grapes (or apple), onions, and herbs.
If using nuts or dried fruit, add them now.
Combine. Add the chicken to the bowl and gently mix until everything is coated. Don’t overmix; keep the texture intact.
Chill briefly. Cover and refrigerate for 15–30 minutes to let flavors blend. This step makes a big difference.
Assemble croissants. Split croissants, add a lettuce leaf to create a barrier, then spoon in a generous mound of chicken salad.
Serve. Enjoy right away, or wrap and keep chilled until lunchtime.
Add a pickle or chips on the side if you like.