Place the steak in the freezer for an hour to ensure it is cold and firm; this helps you cut thin slices. Thinly slice the steak.
Peel and dice the onion.
Preheat a flat-top grill or griddle to a medium-high heat.
Add the butter to the preheated flat-top grill, then cook the diced onion. Remove and put aside until needed.
Add more butter to the grill, then cook the thinly sliced steak. Season to taste with salt and pepper. As it cooks, break the meat into smaller pieces. Cook in batches if needed.
Return the onion and any early cooked batches of the steak to the grill. Combine and heat.
Toast the inside of the bread rolls on the flat-top grill. Put aside until needed.
Divide the steak and onion mix into four piles shaped into lines the size of the rolls, place 3 slices of cheese on each pile, then place a toasted roll over this.
Using a large spatula, remove the Philly Cheese Steak from the flat top grill and close the roll to serve.