In a bowl, combine the drained tuna, mayonnaise, Dijon mustard, chopped olives, chopped parsley, salt, and black pepper. Mix well.
Cut the rolls or baguettes in half. Optionally, toast them lightly for added texture.
Spread the tuna mixture evenly on the bottom half of each roll. Top the tuna mixture with red onion, tomato, and lettuce. Place the other half of the roll on top.
Serve: Slice the bocadillo in half and enjoy your delicious bocadillo de atún with chips, pickle, and pink lemonade.