Classic Tuna Niçoise Sandwich

Slice the baguette lengthwise, but not all the way through, creating a hinge.

In a small bowl, combine the drained tuna, mayonnaise, and Dijon mustard. Mix well until thoroughly combined.

Spread the tuna mixture evenly onto both cut sides of the baguette.

Arrange the halved Niçoise olives, halved cherry tomatoes, and sliced red onion over the tuna mixture on one side of the baguette.

Place the butter lettuce leaves on top of the vegetables.

Gently close the baguette to form the sandwich. Cut the baguette into two equal portions for serving.