In a pestle and mortar, crush peppercorns and garlic with a pinch of salt until you have a smooth-ish paste, then stir in the vinegar. Sit the beef in a non-metallic dish, then rub all over with the paste. Leave in the fridge for a few hrs, but no longer.
To cook, place a griddle pan over a very hot heat. Rub the marinade off the meat, then season with a little more salt. Cook meat until charred on each side – about 5 mins on each side for rare; if the cut is very thick, you may want to roast it in a hot oven for 5 mins after searing. Lift onto a chopping board, then rest for 5 mins before carving into slices and serving with chips, if you like.