Prepare all ingredients as listed above.
In a bowl, combine the drained tuna, mayonnaise, Dijon mustard, chopped olives, chopped parsley, salt, and black pepper. Mix well.
Optional: Toast the sub rolls.
Layer the bottom half of each roll with shredded lettuce, sliced tomato, sliced red onion, and the tuna salad mixture. Add additional olives if desired.
Place the other half of the roll on top and cut each sandwich in half.
Prepare the chia fresca: combine all the ingredients together. Stir well and let sit for 5 minutes to allow the chia seeds to bloom.
Serve the bocadillo de atún with potato chips, pickle spears, and chia fresca.