Dice the red onion into fine pieces and set aside. Dice the red pepper into small, uniform pieces and set aside. Pit the kalamata olives if not already done, then halve them. Chop the fresh basil into thin ribbons. Squeeze the lemon juice fresh and measure out 3 tablespoons—I find fresh lemon juice makes a noticeable difference in brightness compared to bottled, so it's worth the extra minute.

Drain the tuna packed in olive oil into a bowl, reserving about 1 tablespoon of the oil. Add the drained tuna to the bowl and break it apart gently with a fork to create a flaky texture without turning it into a paste. Add the mayonnaise to the tuna and stir to combine evenly. I prefer to mix the mayo with the tuna first before adding the other ingredients so it coats everything uniformly and creates a creamy base.

Add the diced red onion, diced red pepper, halved kalamata olives, and chopped basil to the tuna mixture from Step 2. Pour in the fresh lemon juice and sprinkle with black pepper. Gently fold everything together with a fork until all ingredients are evenly distributed and the flavors are well combined. Taste and adjust seasoning if needed—the lemon juice and olives provide most of the flavor, so go easy on any additional salt.

Lay out 4 slices of bread on a clean surface. Layer romaine lettuce on each slice, making sure the leaves cover the bread evenly—this prevents the bread from getting soggy and adds a crisp texture. Divide the tuna salad from Step 3 evenly among the four bread slices, spreading it generously over the lettuce layer. Top each with another slice of bread and press gently to hold everything together. Slice each sandwich diagonally if desired and serve immediately.