The turkey sandwich is the sandwich with nowhere to hide. Sliced turkey breast, lettuce, tomato, and mayonnaise on plain bread is the baseline against which more ambitious sandwiches are measured, and it is hard precisely because there is nothing in it to cover a weak part. Turkey breast is the leanest, mildest, least seasoned filling at the lunch counter. The defining fact of the turkey sandwich is that its quality is decided almost entirely by three or four ingredients in honest proportion, because none of them is loud enough to rescue the others.
The craft is in the slice and the moisture management. There are two turkeys, and they make two different sandwiches: deli turkey, sliced thin off a machine, stacked enough to have presence without going to a wad; and roast turkey, carved off a bird, where the grain matters and a thick clean cut stays tender while a ragged one turns to thread. Either way the meat brings little fat and little salt, so the mayonnaise is structural as much as it is flavor: spread to both slices it seals the crumb against the tomato and supplies the richness the turkey lacks. The lettuce is the cool crunch the soft filling needs, the tomato is salted and is the thing most likely to flood the bread, and the bread itself is chosen to disappear, soft enough not to fight a delicate meat. Get the ratio of mayonnaise to meat wrong, or let the tomato soak the slice, and the whole thing reads as plain and damp; get it right and the restraint is the point.
The variations are small and mostly honest. Cranberry and stuffing turn it into the Thanksgiving leftover build, a sweeter and wetter register. Bacon and a third slice of toast brace it into a club. A hot version ladles gravy over sliced roast turkey and abandons the hand for a fork. Smoked turkey shifts it toward the barbecue counter entirely. Each of those is a codified build with its own rules, and each deserves its own article rather than being crowded in here.