Place the steak in the freezer for 1 hour before slicing. This makes it easier to get thin slices.
Trim off any large chunks of fat from the steak and set them aside.
Heat the griddle to 300–325°F for juicy cheesesteaks. My griddle runs hot, so I preheat all four burners on low, then turn the outside two off.
Place the reserved fat on the griddle and let it start rendering. If you didn’t cut off any fat, you can use a little oil instead.
Add the onions to the rendered fat or oil. Sauté them low and slow while the griddle continues warming up—about 10 minutes.
Spread the sliced beef on the griddle and use your spatula to shred it apart as it cooks.
Once the beef is cooked through, mix in the onions, then divide the mixture into 2 or 3 piles.
Season each pile with salt and pepper.
Top each pile with two slices of cheese or a spoonful of Cheese Whiz, then place the buns on top to let them steam.
Slide your spatula under each pile and flip it into the bun.
Finish with an extra drizzle of Cheese Whiz, then wrap each sandwich in foil.
Wrapping in foil is a must—it helps steam the bread and makes it soft and pillowy.