In a mixing bowl, combine the cooked chicken, mayonnaise, celery, red onion, Dijon mustard, and grapes if using. Mix well.

Season with salt and pepper to taste.

Lay out the slices of bread on a clean surface.

Spread a generous amount of the chicken salad mixture onto half of the bread slices.

Top with lettuce and tomato slices if desired, then place the remaining bread slices on top to form sandwiches.

Cut sandwiches in half and serve immediately.