Season the beef patties with salt and pepper. Cook them on a cast iron griddle over medium-high heat to your desired doneness. I cook them to medium, about 3 minutes per side. You can also grill them.
Transfer the cooked patties to a plate and allow them to rest, loosely covered in foil, for 5 minutes.
While the meat is resting, assemble the remaining ingredients.
To assemble the "sandwiches," grab two plates. Place one cooked patty on each plate. Spread it with mustard (or use mayo if you'd like). On top of the mustard, layer the lettuce, onion, tomato, cheese, and then top with another cooked patty. Serve immediately, with extra napkins. :)