Chicken Salad Croissant Sandwich

Name: Chicken Salad Croissant Sandwich
Ingredients: ½ cup mayonnaise (or half mayo and half greek yogurt); 1 tsp dijon mustard; 1 tsp lemon zest; 2 tbsp lemon juice (freshly squeezed); ½ tsp kosher salt; ¼ tsp black pepper; 2 cups diced cooked chicken; 1 crisp apple (cored and diced, honey crispy, granny apple, fuji apple); ½ cup seedless grapes (halved); ½ cup chopped walnuts; ⅔ cup celery stalks (about 2 medium celery stalks, diced); 1 small shallot (minced); 4-8 butter or romaine lettuce leaves; 4 croissants (sliced in half lengthwise)