Place the eggs in a boiling pot and cover with water. Bring to a boil, then cover and remove from heat. Let sit for 10 minutes.

Transfer the eggs to an ice bath to cool for a few minutes, then peel and chop them into small pieces.

In a mixing bowl, combine the chopped eggs, mayonnaise, Dijon mustard, soy sauce, green onions, salt, and pepper. Mix until well combined.

Butter one side of each slice of bread. Toast the bread in a skillet or toaster until golden brown.

Spread a generous amount of egg salad on the unbuttered side of four slices of bread. Top with lettuce leaves if using, then place the remaining slices of bread on top, buttered side up.

Cut the sandwiches in half and serve immediately.