The tavern sandwich is named for the room it is served in, not for anything in it, and that is the most important thing to understand about it. In Iowa it is the loose pile of seasoned, unbound ground beef on a soft bun that elsewhere goes by other names, but the local word ties it to the town tavern: the small-town counter where it is scooped to order, eaten in a few minutes, and ordered by the half dozen to take home. The sandwich is inseparable from that setting. It is counter food sized and priced and served for a room where people stand or perch, which is why it is small, why it comes wrapped in waxed paper, and why a tavern measures its day in how many it scooped.
The craft follows from that service context. The beef is browned and broken fine, kept loose and moist on its own rendered fat, never pressed into a patty and never sauced into a wet filling, because a tavern has to produce it fast and consistently from a warmer all day. It is held in a steam well so any order is a single scoop onto a plain soft bun, and the bun is chosen to soak the fat and compress to the mound rather than fight it. The dress is deliberately minimal, mustard, pickle, raw onion, applied the same way every time so the sandwich reads identically whether it is the first of the day or the two hundredth. The spoon that comes with it is not an apology for a sandwich that will not hold; it is the tool the counter hands you because the format never pretended the pile would stay put, and that honesty is part of the room's character.
The variations are a matter of which counter you are standing at rather than a different sandwich. A cheese tavern melts a slice over the warm beef; a particular town's tavern guards its exact seasoning and dress as a local signature. The generic loose meat sandwich is the same idea with the room stripped out of the name, and the chain reading codifies it to a single fixed recipe; the sauced Sloppy Joe is the cousin that crosses into a bound filling and reads as its own thing. Each of those deserves its own article rather than being crowded in here.