The hot open-face roast beef gives up the second slice of bread and, with it, any pretense of being held in the hand. Sliced roast beef is laid over a single slice of soft white bread, the whole thing is flooded with brown gravy, and a scoop of mashed potatoes goes alongside, also under gravy, eaten with a knife and fork. The defining element is the gravy, not the beef. Anything can be put on white bread; what makes this its own dish is the gravy that soaks the slice into something between bread and dressing, binds the meat to it, and ties the potatoes into the same plate. This is diner economics made into a sandwich: a way to turn a modest amount of roast beef and yesterday's bread into a filling, satisfying lunch special by letting the gravy do the stretching.
The craft is in keeping the base from vanishing before the meal is over and in making the gravy carry the plate. The white bread is soft and plain on purpose, chosen because it should surrender to the gravy, not resist it; the slice is meant to read as a soaked, savory bottom layer rather than as toast holding its shape. The roast beef is sliced thin and laid flat so the gravy reaches all of it and so a modest portion covers the bread completely, which is the quiet economy of the build. The gravy is the structural binder and the dominant flavor at once: brown, well-seasoned, loose enough to soak in and thick enough to coat, and it has to season bland bread and bland potatoes simultaneously, so it is made assertive by necessity. The mashed potatoes are not a side so much as a second canvas for the same gravy, which is what makes the plate read as one composed thing rather than meat and starch sharing a dish. Timing decides it: served promptly, the bread is a pleasing soaked layer; left too long, it is gone before the fork finds it.
The variations keep the single-slice, knife-and-fork, gravy-bound logic and swap the meat. The hot turkey plate runs sliced turkey under the same gravy; the hot meatloaf and hot hamburger plates do the same with a different center. The Louisville Hot Brown belongs to the same family but reaches for a cheese Mornay and a broiler instead of brown gravy, which makes it a distinct sandwich. Each of those deserves its own article rather than being crowded in here.