At Tony Luke's at Oregon and Front Street in Philadelphia
Update: Forgot to put down what it was - think this was pork and broccoli rabe.
Further Update, 12/2013: Just found this recipe: http://www.food.com/recipe/tony-lukes-italian-roast-pork-sandwich-the-real-deal-109581 Check out a Philly cheesesteak song here: http://www.dropbox.com/s/zvgxpfkxcpal0r4/CheeseSteakSong4Web.mp3?dl=0