1. Preheat the grill to medium-high heat.
2. Season the chicken breasts with salt and pepper.
3. Grill the chicken for 5-7 minutes on each side, or until fully cooked.
4. Let the chicken cool, then chop it into small pieces.
5. In a mixing bowl, combine the chopped chicken, mayonnaise, Dijon mustard, celery, red onion, salt, and pepper.
6. Spread the chicken salad mixture onto four slices of toasted bread.
7. Top with shredded lettuce and tomato slices.
8. Place the remaining slices of toasted bread on top to form sandwiches.