The panino con tonno e fagioli is a Tuscan salad turned into a sandwich, and what defines it is the bean, not the fish. Tonno e fagioli on the plate is canned tuna, cannellini beans, raw onion, and olive oil, and the cannellini is what gives the dish its body: soft, starchy, mild, it carries the oil and quiets the salt of the tuna in a way no leaf could. Moved into bread, that role becomes structural. The beans bind the loose mixture, hold it together against the crumb, and turn what would be a wet, sliding tuna filling into something with enough mass to behave between two slices. The fish supplies the salt and depth; the bean supplies the substance.
The craft is moisture control, because every component here runs damp. The cannellini are used whole and well drained, never mashed, so they give the filling structure rather than turning it into a spread that would soak the bread; some are left slightly broken so the oil and starch help bind the rest. The tuna is flaked in for savour and not over-drained, since its oil is part of the dressing along with a generous thread of good olive oil. Raw onion is the sharp counter that keeps the bean-and-oil base from reading flat, sliced thin and used in measure. The bread is chosen sturdy, a country loaf or a firm roll, and is eaten soon after building, before the beans give up their moisture and the crumb softens under them.
The variations are about which element leads and what acid joins it: the onion-forward build, the version sharpened with a little vinegar or lemon, the one weighted more to bean than to tuna for a milder, starchier bite. Each is a different ratio of the same Tuscan trio on bread, and each deserves its own article rather than being crowded in here.