Place the eggs in a single layer in a saucepan and cover with cold water by about 1 inch.
Bring the water to a boil over medium-high heat. Once boiling, cover the saucepan, remove from heat, and let the eggs sit for 10 minutes.
Drain the hot water and transfer the eggs to a bowl of ice water to cool for 5 minutes.
Peel the cooled eggs and chop them coarsely.
In a medium bowl, combine the mayonnaise, Dijon mustard, lemon juice, salt, and black pepper. Mix well.
Add the chopped eggs, celery, red onion, and chives to the bowl. Gently fold to combine all ingredients evenly.
Cut the baguette into 4 equal pieces. Slice each piece lengthwise without cutting all the way through, creating a pocket.
Place one large lettuce leaf inside each baguette piece.
Divide the egg salad evenly among the four baguette pieces, spooning it onto the lettuce.
Close the sandwiches and serve immediately or wrap tightly and refrigerate until ready to serve.