Peel the hard-boiled eggs and chop them into small pieces.
In a mixing bowl, combine the chopped eggs with mayonnaise and Dijon mustard.
Add the chopped dill, celery, and red onion to the egg mixture.
Season with salt and pepper to taste.
Mix all the ingredients until well combined.
Lay a leaf of lettuce on one slice of bread.
Spoon the egg salad mixture over the lettuce.
Add tomato slices on top if desired.
Place the second slice of bread on top to complete the sandwich.
Cut the sandwich in half and serve immediately.