Tomato is the wettest thing you can put against tuna in a soft crustless triangle, and the tramezzino tonno e pomodoro is a build defined by managing that water rather than ignoring it. The tuna is oil-packed and flaked, bound with mayonnaise into a savoury mass that fills the corners and films the crumb. The tomato, sliced thin, brings a sweet-acid juiciness and a cool fresh lift that the dense bound tuna lacks entirely. That freshness is the reason the sandwich exists, and the moisture it carries is the reason it is hard to do well. The tuna gives body and salt, the bread gives the soft pale frame, and the tomato cuts through both with brightness and a faint vegetal crunch at the skin. Without the tomato this is the plain tuna tramezzino; without the tuna the tomato slips around in an empty soft shell. The two are arranged to need each other along the line where richness meets acid and water.
A good one is mostly a study in keeping the tomato from drowning the bread. The loaf is soft white pancarrè, fresh that day, soft to a thumb, all crust trimmed so only the tender crumb remains. The tuna is drained well and bound with just enough mayonnaise to cohere, kept a touch firmer here because it will be sitting next to a wet ingredient. The tomato is ripe but firm, sliced thin, the seeds and watery pulp scooped out, and the slices salted briefly and blotted dry so they shed their juice on the board and not into the crumb. The mayonnaise does double duty, binding the tuna and laying a sealing film on the inner faces of the bread so the tomato's residual moisture meets fat rather than bare crumb. The build is mounded toward the centre so the triangle domes, fullest in the middle and thin at the cut. A sloppy version lays wet unsalted tomato straight onto bread and serves a grey soggy corner within minutes; a careful one seeds and blots the tomato, seals the bread, centres the dome, and cuts a diagonal that still holds its shape.
The close cousins each deserve their own article rather than being crowded in here. Add a leaf of lettuce and the build gains crunch and changes the moisture math again. Fold chopped egg into the tuna and the sandwich moves into the egg-and-tuna register. Swap the tomato for olives or marinated artichokes and the wet sweet lift becomes a dry briny one. Take the tomato out entirely and you are back at the unadorned tuna tramezzino, the baseline this version is measured against and best read on its own terms first.