The spinach build is the gentlest reading of the Philadelphia roast pork, and the choice of spinach over broccoli rabe is the entire point. It is the same slow-roasted, garlic-and-fennel pork shoulder, sliced thin and piled on a long Italian roll with provolone, but the green here is sautéed spinach instead of bitter rabe. Spinach brings body and a soft mineral note without the aggressive bitterness, so the sandwich reads sweeter and rounder, with the seasoned pork rather than a sharp vegetable doing the leading. People who find the rabe version too bitter order this one on purpose; it is not a lesser sandwich, it is a different argument about how much the green should fight the meat.
The craft is in the roast and the wilt. The shoulder is cooked low and long until it shreds into tender, jus-heavy strands, then sliced or chopped so it stays pliable rather than seizing in the pile, with the pan juices kept back to moisten the meat or the cut faces of the roll. The spinach is sautéed fast in garlic and oil and wilted down hard, then drained, because spinach holds water and a wet handful would flood the roll and turn the bread to paste before the sandwich reached the hand. Drained well, it goes on as a soft, garlicky layer that cushions the pork rather than countering it. Provolone, often the milder rather than the sharpest grade here to match the gentler profile, is set against the hot meat so it half melts and binds the load. The roll is the Philadelphia long roll, tender inside with a crust that can carry a heavy, slightly wet, three-part filling without folding at the middle. Built right it is warm, soft, and savory in a single register, assembled fast at a counter and eaten before the jus soaks through.
The variations are the rest of the roast pork family. The canonical build uses garlicky broccoli rabe for real bitterness; the long-hots version trades the green entirely for fried hot peppers and heat. Both of those deserve their own articles rather than being crowded in here, as does the cheesesteak this sandwich shares a roll and a city with.