The usual "Philadelphia cheesesteak" outside of the immediate area consists of slices of roast beef on a long sandwich roll, with miscellaneous cheese (usually Swiss or American), raw onion, maybe sauteed mushrooms and bell pepper, lettuce, and tomato.
The real thing is quite different. Here's how you do it:
Take a slab of meat. A chicken breast will work, if you're a heathen or trying to be healthy, but the canonical version is a slice of frozen beef about the size of the roll and a quarter inch or so thick. Drop it on the grill. Take your sharp-edged metal spatula and shred it while it begins to cook. If you want onions, take some chopped onion and drop it into the meat. Cook, stirring to make sure it's cooked evenly. This will go quickly. When it's almost done, layer slices of cheese (canonically, Provolone) on top, and let it melt into the shredded meat. Spread mayo or ketchup on the inside of the roll as desired. Scoop up the meat with your spatula and place in the roll. Eat.
There are those who will argue that the kind of crusty Italian bread you find in Philadelphia is an essential component. It certainly makes things even better, but the folks at the cafeteria here use a standard submarine sandwich roll, and it worked well enough. The key is the cooking method, which is designed to go quickly: a cheesesteak is fast food, and the kind of lunch wagon you see in Philadelphia depends on high volume to make money off of its inexpensive sandwiches.