The Hot Brown gives up the second slice of bread, and with it the idea that a sandwich is held in the hand. It is an open-faced build: sliced roast turkey laid over a single slice of toast, the whole thing flooded with Mornay, a cheese-enriched béchamel, then topped with bacon and tomato and run under a broiler until the sauce blisters and browns. The defining element is the Mornay, not the turkey. Anything can be put on toast; what makes this its own dish is the sauce that coats it, soaks down into the bread, and binds turkey, bacon, and toast into a single broiled plate eaten with a knife and fork. It is the most composed member of its family: counter food given real kitchen technique.
The craft is in keeping the base from dissolving before the meal is over. The toast is firm, often crusts-on and griddled or toasted hard, because it has to take a heavy ladle of Mornay without turning to mush in the time it takes to broil and serve. The sauce is the structural binder as much as the flavor: rich enough to coat, loose enough to soak in, finished with a hard cheese so it browns rather than just melting. The turkey is the substance under it, sliced and laid flat so the sauce reaches every part. Bacon goes on top for salt and crispness against all that richness, and tomato adds the only acid in the build, cutting a sauce that would otherwise overwhelm. Timing decides everything: the broiler has to set the top and brown the cheese before the toast underneath surrenders. A hotel kitchen can build these in oval gratin dishes and fire them in a salamander to order, all evening.
The variations keep the single-slice, knife-and-fork, sauce-bound logic. Ham appears alongside or instead of the turkey in some builds, and the proportion of bacon and tomato shifts from kitchen to kitchen. The Hot Brown belongs to the same family as the Springfield horseshoe, which buries meat and fries under cheese sauce, and the diner hot turkey plate, which floods toast and turkey with gravy instead of Mornay. Those are their own sandwiches with their own rules and deserve proper articles of their own rather than being crowded in here.