Prepare all ingredients as listed. Drain the tuna thoroughly.
In a medium bowl, combine the tuna, mayonnaise, Dijon mustard, sliced olives, chopped parsley, salt, and black pepper. Mix well.
Optional: Lightly toast the sub rolls if desired.
Layer the bottom halves of the rolls with shredded lettuce, sliced tomato, and sliced red onion. Spread the tuna salad mixture evenly over the vegetables. Add extra olives if desired.
Place the top halves of the rolls on the sandwiches and cut each in half.
For the chia fresca, combine all the ingredients together. Stir well and let sit for 5 minutes for the chia seeds to bloom.
Serve the bocadillo de atún halves with potato chips, pickle spears, and sparkling chia fresca.