Brush the entire outside of the whole, uncut ciabatta loaf with olive oil. Place it in a hot panini press or skillet and press for 2–3 minutes, until flattened and lightly crisped. Remove and let cool slightly.

In a bowl, combine tuna, mayonnaise, sriracha, Dijon (if using), salt, and pepper. Mix until creamy and well combined.

Once cooled slightly, slice the ciabatta open lengthwise, like a book (do not cut it into portions yet).

Bottom side: spread basil pesto, then layer sliced Roma tomato and thin red onion.

Top side: mash ½ avocado, then spread the tuna mixture over it.

Close the sandwich gently.

Cut the full sandwich crosswise into halves or portions and serve immediately.