Lean meat is the challenge the tramezzino al pollo is built to answer: cooked chicken is mild and dry on its own, and inside a soft crustless white triangle it needs the mayonnaise bind more than almost any other filling does. The chicken here is poached or roasted breast, shredded or diced into a cool, tender, faintly savoury mixture that holds in the dome. The crumb is airy, sweet, and close to flavourless by design. Put dry meat against bland bread and the result is two quiet things with nothing joining them, which is exactly why the bind is structural rather than decorative. The mayonnaise carries the flavour and the moisture the chicken lacks, glues the loose shreds into a mass, and seals the crumb so the whole soft construction holds in the hand. Without it the sandwich is dry on dry; with it the lean meat reads as rich and composed.
A good one lives or dies on moisture. The loaf is a fine soft white sandwich bread, baked that day, the crust trimmed flush off every side so only the tender interior remains, and the slices kept under a damp cloth so the edges never go dry or stiff. The chicken is cooked just to done and no further, since overcooked breast turns stringy and refuses to absorb the dressing, then pulled or chopped small and folded into enough mayonnaise to coat every piece and to film the inner face of the bread. That film is the waterproofing: even a fairly dry filling can leach a little, and the sealed crumb is what keeps the bottom slice from going pasty over the hour it sits in a bar case. The mixture is mounded toward the middle so the cut triangle stands tall with a domed centre and a thin pinched edge, and a careless one is plain to see, dry and underdressed so it crumbles apart, or overfilled and damp so the bread bruises and the filling slides.
The variations stay on the lean-meat logic and swap one element. There is the build that adds diced celery or pickle for crunch and acid against the soft mixture, the curry-tinted version that warms the mayonnaise with spice, and the one that joins the chicken with thin egg or a leaf of lettuce for more body and freshness. Each of those is the same dressed chicken in a soft dome with a single change, and each deserves its own article rather than being crowded in here.