Chicken Salad Croissant Sandwiches

Name: Chicken Salad Croissant Sandwiches
Ingredients: 3 chicken breasts (split, bone-in, skin-on (about 2 ½ pounds or 5 cups chopped or shredded cooked chicken)); Olive oil; Salt; Coarsely-ground black pepper; 1 ¼ cups mayonnaise; 2 tablespoons Dijon mustard; 2 tablespoons white vinegar or red wine vinegar; 2 ½ tablespoons sugar; 1 ½ cups red seedless grapes (sliced in half); 1 cup chopped celery; ⅝ cup sliced scallions (divided ½ cup + 2 tablespoons); ½ cup sliced almonds; ⅓ cup dried cranberries; 4-6 croissants (sliced in half); green leaf lettuce and tomato slices for serving (optional)