Origin. This follows the Mediterranean tuna sandwich tradition (Italian tonno panini, the Niçoise pan bagnat) rather than the American deli one. Deli tuna salad uses water-packed tuna and adds fat back with mayo; oil-packed tuna already has its fat, so this style uses olive oil, lemon, raw onion, and something briny instead.
Flake the tuna with its oil, the capers, a squeeze of lemon, a little extra olive oil, salt, and pepper.
Split the roll. Pile on the tuna, onion, peppers, and lettuce if using.
Close and eat, or press (see Notes).
Use oil-packed tuna; water-packed changes the sandwich and needs mayo to compensate.
Pressing. A panini press works well, especially if the roll is past its prime. Blot the peppers first so the bread doesn't steam, keep the heat medium so the inside warms without melting into a tuna melt, and add lettuce after pressing or not at all.
Variations: