Chop cooked chicken into small pieces or shred it. If you want a deli style texture, chop finely. If you prefer chunkier chicken salad, keep larger pieces.
In a bowl, stir together mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Taste and adjust. This is your flavor base.
Stir in diced celery, onion, relish, and dill.
Add chicken and fold until evenly coated. Do not overmix if you want some texture. Chll
Slice the croissants in half and add lettuce, chicken salad, and tomato if you like. Slice and serve.