Chop 3½ cups cooked chicken. Dice 2 stalks celery and slice 2 green onions.
Combine ¾ cup mayonnaise, ½ cup sour cream, 2 teaspoons Montreal chicken seasoning, celery, and green onion. Mix well. Add chicken and gently stir. Assemble as sandwiches with 8 lettuce leaves on 8 croissants.