The panino con tonno is the workhorse fish sandwich of the Italian bar, and what defines it is not the fish so much as the oil it sits in. The tuna here is tonno sott'olio, canned and packed under olive oil, and that oil is the point: it is the dressing, the binder, and the seasoning all at once. Drained and flaked onto bread, the tuna is salty, dense, and faintly metallic on its own; the oil it carries softens that edge, slicks the crumb, and turns a pile of preserved fish into something that reads as composed. This is the sandwich's whole logic. The cure has already done the cooking, the oil does the dressing, and the bread does nothing but hold the result steady.
The craft is in restraint against salt and oil, because both run high and both can flood the bread. Good tinned tuna is lifted out in large flakes rather than mashed to a paste, so the texture stays meaty instead of going to mush, and it is not drained completely dry: a little of its oil is left to gloss the crumb without soaking through it. A plain crusted roll or a length of ciabatta is the usual carrier, sturdy enough that the oil greases it rather than collapsing it. Almost nothing else is needed. A few capers, a thin slice of onion, or a leaf or two adds a sharp counter to the richness, but the build that holds together cleanest is close to bare, the tuna and its oil left to be the entire statement rather than buried under mayonnaise or salad.
The variations are about what single thing is allowed to join the fish: the version with capers and onion for an acid lift, the one with a few black olives, the tomato-and-tuna build that brings its own moisture and wants the bread firmer still. Each of those is a different small partner against the same oiled tuna, and each deserves its own article rather than being crowded in here.