1. Grill the chicken breasts until fully cooked, then chop into small pieces.
2. In a mixing bowl, combine the chopped chicken, mayonnaise, celery, red onion, Dijon mustard, salt, and black pepper.
3. Mix well until all ingredients are evenly distributed.
4. Toast the bread slices.
5. Assemble the sandwiches by placing a lettuce leaf on a slice of toasted bread, adding a generous scoop of chicken salad, topping with tomato slices, and covering with another slice of toasted bread.