In a mixing bowl, combine the cooked chicken, mayonnaise, celery, grapes, almonds, dijon mustard, lemon juice, salt, and black pepper. Mix until well combined.

Taste and adjust seasoning if necessary.

Slice the croissants in half horizontally.

Spoon a generous amount of chicken salad onto the bottom half of each croissant.

Add lettuce and tomato slices if desired, then place the top half of the croissant on the sandwich.

Arrange the sandwiches on a serving platter and serve immediately.