Hard boil the eggs by boiling them in water for 10-12 minutes.
Transfer eggs to an ice bath for 5 minutes to cool.
Peel the cooled eggs and chop them into small pieces.
In a mixing bowl, combine the chopped eggs, mayonnaise, and Dijon mustard.
Add the chopped chives, salt, and pepper to the mixture.
Stir until all ingredients are well combined.
Toast the bread slices lightly if desired.
Spread a generous amount of egg salad on one slice of bread.
Add lettuce leaves and tomato slices on top of the egg salad.
Top with the second slice of bread, cut in half, and serve immediately.