Sliced beef on garlic bread is a St. Louis roast beef sandwich defined by two local substitutions that change everything: the bread is garlic bread, and the cheese is Provel. Neither is a flourish. Garlic bread, buttered and toasted with garlic worked into it, replaces the neutral roll a roast beef sandwich usually leans on, so the carrier is assertive instead of recessive and seasons the sandwich rather than just holding it. Provel is a St. Louis processed cheese blend engineered to go soft and gooey at low heat without stringing or splitting, which lets it coat warm sliced beef in a smooth layer rather than draping it in slices. Together they make a roast beef sandwich that tastes specifically of this city and nowhere else.
The craft is in the toast and the melt working as a system. The bread is toasted firm enough to take warm, slightly moist beef without going limp, the garlic butter crisping into the surface so it carries through every bite the way jus carries through an Italian beef. The beef is sliced thin against the grain so a warm pile stays tender rather than turning to rope, and the Provel is melted onto or under it so the cheese flows into the meat instead of sitting on it, the same fusion logic a flat-top cheeseburger relies on. The balance the sandwich has to strike is richness against richness: garlic butter, fatty beef, and a soft melting cheese, held in check only by the structure of the toasted bread and the thinness of the slicing. Done right it is cohesive and rich; done carelessly it is heavy and slack.
The variations stay close to the beef-cheese-garlic-bread idea and mostly change the heat and the cut. Some builds serve it warm and open, others closed and pressed; some add jus on the side, pulling it toward its French dip and Italian beef relatives. It belongs to the wet, thin-sliced roast beef family alongside the Chicago Italian beef, the Los Angeles French dip, and beef on weck, each a regional reading of the same idea. Those deserve their own articles rather than being crowded in here.