Clementes, a butcher shop on Avenue T in Brooklyn has an unbelievable roast beef hero. I got one with provolone and gravy, and I'm dying to go back. The roast beef is rare with an rich, big flavor. The gravy is just enough to pump up the flavor and not make the hero soggy.
Looks like the make their own mozzarella too, though I didn't try it.
Clementes: 188 Avenue T