Prep all ingredients per the instructions above.
In a bowl, combine the tuna, mayonnaise, Dijon mustard, chopped olives, chopped parsley, salt, and black pepper. Mix well until evenly combined.
Optional: Toast the sub rolls in a toaster oven until lightly golden.
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. Cut in half if preferred.
Combine water, lemon juice, chia seeds, and fresh mint. Stir well and let sit for 5 minutes to allow the chia seeds to hydrate. Serve over ice.
Serve the bocadillo de atún with carrots and hummus, pickle, watermelon, and lemon mint chia fresca.