Place the eggs in a saucepan and cover them with water. Bring to a boil over medium-high heat. Once boiling, cover the pot, remove it from heat, and let it sit for 12 minutes.

After 12 minutes, transfer the eggs to a bowl of ice water and let them cool for about 5 minutes.

Peel the cooled eggs and chop them into small pieces.

In a mixing bowl, combine the chopped eggs, mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Mix until well combined.

Fold in the chopped celery and red onion until evenly distributed.

Toast the bread slices if desired.

Spread the egg salad mixture onto two slices of bread, add lettuce leaves if using, and top with the remaining slices. Cut in half and serve.