Use the back of a fork to mash the egg yolk together with the mustard until a thick paste forms.
Stir in the chopped scallions and chopped egg white, ensuring they are well combined.
Add salt and pepper to the mixture according to your taste preferences.
Take 2 slices of bread and divide the egg mixture, spreading it evenly on top of each slice.
Place lettuce leaves over the egg mixture on one slice of bread.
Cover the lettuce with the remaining slices of bread to complete 2 sandwiches.