Boil the eggs in water for 8–10 minutes until they are hard-boiled.
Peel the eggs once cooled and place them in a bowl.
Mash the boiled eggs with a fork until crumbly but not completely smooth.
Add mayonnaise, mustard (if using), salt, and black pepper, then mix until creamy.
Lightly butter the bread slices and toast them if you prefer extra crunch.
Spread the egg mayo mixture evenly over one slice of bread.
Add lettuce leaves or cucumber slices if desired for freshness.
Place the second slice of bread on top to close the sandwich.
Cut the sandwich diagonally or into halves for serving.
Serve immediately and enjoy your creamy egg mayo sandwich.