The pastrami burger puts two finished sandwiches on top of each other and treats the seam between them as the recipe. A griddled beef patty with melted cheese is built as a cheeseburger would be, and then a pile of hot, sliced pastrami is laid over the cheese before the bun goes on. The defining decision is that the pastrami sits against the molten cheese, not against the bun, so the cheese fuses the cured meat to the patty and the two proteins arrive as one bonded layer rather than a burger wearing a hat. The fusion is the whole sandwich; without the cheese between them the build is just a patty and some deli meat sharing a roll.
The craft is in heat and order. The patty is seared on a flat-top for a hard crust, and the cheese, a melting slice chosen to flow rather than split, is laid on while the patty is still on the steel so it is fully molten at assembly. The pastrami is warmed and sliced so it is slack and foldable, then set onto that live cheese the instant the bun is closed, which is when the cheese grips both surfaces and locks the stack. The bun is deliberately soft so it compresses to a tall, rich filling instead of fighting it, and the cool, acidic dressing, pickle, raw onion, a squirt of mustard and a special sauce, is doing structural flavor work: it is the sharp counter that keeps two layers of fatty cured-and-griddled meat from reading as one heavy note. Built right it is a controlled excess, the cheese holding a stack that should fall apart but does not.
The pastrami burger sits in the American burger family, where the regional dialects argue about the patty rather than what is piled on it. The smashed style drives a thin crisp-edged patty on the flat-top; the Oklahoma onion build fries shaved onion into that crust; the Connecticut steamed version cooks patty and cheese soft in a vapor box; the Juicy Lucy seals the cheese inside the meat. Each of those is its own sandwich and deserves its own article rather than being crowded in here.