The mushroom cheesesteak is the Philadelphia cheesesteak with a soft, earthy layer added under the cheese, and that layer changes the sandwich more than its single-ingredient description suggests. Sliced mushrooms are sauteed down on the same flat-top as the beef until they give up their water and concentrate, then folded into the chopped meat before the cheese goes on. The defining thing is not the mushroom as a topping but the mushroom cooked into the pile: it loses its raw structure entirely and becomes a moist, savory mass that threads through the beef, deepening the meat's flavor and changing how the whole filling eats. This is a cheesesteak whose center is wetter, darker, and rounder than the plain build, by design.
It works because the mushroom is treated as part of the meat, not an addition to it. Raw or undercooked mushrooms would flood the sandwich and slacken the roll; cooked down hard on the steel until the water is driven off, they integrate into the chopped beef the way fried onion does, adding moisture and umami without making the pile soupy. The cheese, Cheez Whiz, sliced provolone, or American, still goes on while everything is on the griddle so it melts into the meat-and-mushroom mass rather than onto it, and it carries a heavier load here because the mushroom has added moisture and savor that the cheese has to bind. The long Philadelphia roll plays its usual structural part, tender inside and crusted enough to hold a heavy, fused, now-wetter filling without tearing or dissolving. The order grammar is the cheesesteak's own: wit or witout for onions, and the mushroom layer is itself a standing call at the window.
The variations are small swaps on the same griddle. A mushroom-and-onion build runs both soft layers together; a Swiss-cheese version trades the usual cheeses for the nuttier melt that mushrooms invite; a pizza or pepper steak adds a different variable on top of the same mushroom base. It is one of several codified swaps on a fixed method, alongside the plain and chicken builds, and those deserve their own articles rather than being crowded in here.