Of all the tramezzino fillings, turkey is the one that vanishes fastest without help, and the tramezzino al tacchino is shaped entirely around that fragility. The turkey here is cooked breast, sliced or shredded, exceptionally lean and exceptionally mild, with so little fat and so little salt that on its own it barely registers. The crumb around it is airy, sweet, and close to flavourless by design. Put the palest meat against the palest bread and there is almost nothing there, which is precisely why the mayonnaise bind is doing most of the work. It supplies the moisture and the savour the turkey lacks, glues the loose meat into a coherent mass, and films the crumb so the soft triangle holds in the hand. Without that bind the sandwich is dry on dry and falls apart; with it, the lean meat reads as tender and complete.
The craft is keeping a near-flavourless filling from also being a dry one. 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 stiffen or dry. The turkey is cooked just to done, since a minute too long turns the breast chalky and unable to take the dressing, then pulled or folded loose and worked into enough mayonnaise to coat every piece and to film the inner face of the bread. That film is the waterproofing as much as the seasoning: a lean filling still gives up a little moisture over an hour in a bar case, and the sealed crumb is what keeps the bottom slice from going pasty. The mixture is mounded toward the middle so the cut triangle stands with a domed centre and a thin pinched edge, and a careless one is obvious, underdressed and dry so it crumbles, or thin and flat so the triangle has no presence at all.
The variations stay on the lean-meat logic and swap one element. There is the build that adds a thin layer of cheese for fat and grip against the dry meat, the one that works in lettuce and tomato for freshness and acid, and the version that joins the turkey with a slick of cranberry or mustard for a sharp sweet edge. Each of those is the same dressed turkey in a soft dome with a single change, and each deserves its own article rather than being crowded in here.