The St. Louis-style pizza sandwich is defined by a cheese that behaves like no other cheese on a sandwich. Provel is a processed blend built around the melt, engineered to go liquid and slightly stretchy at low heat and then set into a soft, almost gummy sheet as it cools. That property is the entire reason this works as a folded, handheld thing rather than a slice eaten flat. The cheese does not slide off in a sheet or split into oil the way a long-aged hard cheese would. It coats, binds, and stays put, which is exactly what a sandwich made from pizza needs.
The construction follows from the cheese and from the cracker-thin St. Louis crust it sits on. That crust is unleavened-stiff and snaps rather than folding, so the sandwich move is to take a hot, Provel-laden section, top it with the same fillings a local pizza carries, sausage, the city's signature crumbled style, or vegetables, and close it cheese-to-cheese against a second piece so the melted Provel itself becomes the glue holding two rigid faces together. Built hot and eaten before the cheese fully sets, it bites cleanly: the crust shatters, the Provel pulls slightly, and the fillings stay locked in the cheese matrix instead of falling out the open side. Built cold or reheated, the Provel firms into something denser and chewier, which is a different sandwich that happens to share a name.
This is a strongly local format, and the variations track the local pizza menu more than any sandwich tradition. A heavier sausage load, a vegetable build, a double-cheese version that leans hard into the Provel, a fold rather than a closed pair. The broader open-face and hot-plate sandwiches and the dense field of place-named regional specialties run nearby, and those deserve their own articles rather than being crowded in here.