In New York the meatball sandwich is a parm hero, and the word parm is doing the work: it is a sandwich finished the way a pizzeria finishes things, under a broiler with mozzarella browned on top. The build is not assembled and served. Meatballs in red sauce go into a hero, fresh mozzarella is laid over them in slabs, and the open sandwich goes into the same deck oven that bakes the pies until the cheese melts, pulls, and takes color in spots. That broil is the defining step. It fuses the meatballs, the sauce, and the cheese into one molten mass and gives the hero its blistered, slightly charred lid, which is why a parm hero almost always comes out of a pizzeria rather than a deli.
The craft is in the oven and the bread. The hero is a length of Italian bread with a crust sturdy enough to carry a heavy, saucy, cheese-capped load without collapsing, and it is given a quick toast before the meatballs go in so the crumb does not surrender to the red sauce on contact. The meatballs are a beef or beef-and-pork blend, often simmered in the sauce so they carry its seasoning all the way through, then arranged the length of the roll so the cheese can bridge them. Fresh mozzarella is chosen for the pull and the way it browns unevenly under high heat; a dusting of grated hard cheese over the top sharpens the finish and adds salt. The deck oven's intense top heat does in a couple of minutes what a home broiler struggles to do at all, setting the cheese and crisping the bread's shoulders at the same moment.
The variations track the pizzeria menu. A chicken parm hero runs the same broil with a breaded cutlet; an eggplant parm hero does it meatless; extra sauce on the side acknowledges that the broil pulls moisture out and some eaters want it back. A heavier ricotta layer pushes it toward a baked-ziti register. The New England grinder and the Philadelphia hoagie build the same meatballs under their own local rules, and the national meatball sub flattens the idea into a chain template, and each of those deserves its own article rather than being crowded in here.