Prepare the Tuna Mixture: In a bowl, combine the drained tuna, mayonnaise, Dijon mustard, chopped parsley, salt, and black pepper. Mix well.
Slice and Toast the Bread: Cut the rolls or baguettes in half. Optionally, toast them lightly for added texture.
Assemble the Bocadillo de Atún: 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.
Mix together all of the ingredients for the homemade lemonade.
Serve: Slice the bocadillo in half and enjoy your delicious bocadillo de atún with chips, pickle, and homemade lemonade.