Heat a grill or large grill pan (you can use a regular pan in a pinch) to medium-high with a light layer of oil. Form 4 to 6 patties from meat. You can make them as large or as small as you want—just make them thinner than your average beef patty (flat like a smashed burger, even!) so they cook through quickly.
Place patties on grill or grill pan. Cook for 4 to 5 minutes on each side, depending on how large your patties are.
While patties are cooking, slice onion into rings. You want them to be fairly thin, but don’t stress over it. Break onion rings apart and put them in a bowl with vinegar and a dash of salt. Make sure rings are fully submerged (add more vinegar if needed). Let sit for at least 5 minutes.
When patties are about 1 minute from being done, flip them again so hot side is facing up. Top with 1 or 2 slices of cheese. Place on buns with lettuce, if desired. Top with onions and peppers.
