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