The Mushroom Sandwich here is the Mumbai grilled kind: sautéed spiced mushrooms with cheese, pressed into a hot toasted sandwich. It belongs to the same street-cart family as the grilled Bombay sandwich, but the filling is built around mushrooms cooked down with onion and spice rather than the usual stack of raw and boiled vegetables. The angle is a warm, savory, slightly juicy filling, the mushrooms giving a meaty chew and the cheese binding it into something that holds together when the sandwich is cut.
The build is a cooked filling sealed into bread and grilled. Mushrooms are sliced and sautéed with onion, chilli, and spice until their water has cooked off and they have taken on color; this step is the whole game, because mushrooms that go in wet will steam the bread soggy from the inside. Soft white or brown bread is buttered to the edges and often spread with green chutney. The cooked mushrooms go on, topped with grated cheese, sometimes with a few raw vegetables for crunch, then closed and pressed in a hot grill until the outside is crisp and marked and the cheese has melted to glue the filling. Good execution shows a deeply browned, firm exterior, a hot filling where the mushrooms are browned and not weeping, and melted cheese holding clean-cut halves together. Sloppy versions are soggy from underdrained mushrooms or skipped butter, bland from underspiced filling, or burned outside while the cheese inside never melts because the press was too hot and too fast.
Variations track the filling balance and bread. Some carts go cheese-heavy for a molten, rich result; others keep mushrooms dominant and the cheese as a binder. Added capsicum and onion, a chilli sauce, or a chaat-masala dusting are common. The plain and masala Bombay sandwiches it sits beside are their own well-defined things and deserve their own article rather than being crowded in here. The constant is the format: spiced sautéed mushrooms and melted cheese, sealed in buttered bread and pressed hot.