In a large mixing bowl, combine the cooked chicken, mayonnaise, Greek yogurt, Dijon mustard, and lemon juice. Mix well until the chicken is evenly coated.
Add the chopped celery, red onion, grapes, and nuts (if using) to the chicken mixture. Stir gently to combine.
Season with salt and pepper to taste. Adjust the seasoning as needed.
Cover the bowl with plastic wrap and refrigerate the chicken salad for at least 30 minutes to allow the flavors to meld.
When ready to serve, slice the croissants in half horizontally. Place a lettuce leaf on the bottom half of each croissant.
Spoon a generous amount of the chicken salad onto the lettuce, then place the top half of the croissant on top.
Serve immediately or refrigerate until ready to serve.