Remove any excess moisture by patting the joint all over with kitchen paper then score the fat on the joint in a criss cross pattern.
Place the meat, fat side down in a large frying pan or skillet and cook over a medium heat for about 5 minutes until all the fat has a golden colour.
Turn the meat over and sear the bottom and sides for a few minutes.
Peel and slice a large onion into rings about 1cm in width and lay the slices on a heavy based roasting tin with the meat on top. Season with salt and pepper.
Cook at 220C/475F/200FAN/Gas9 for 20 minutes then reduce to 190C/375F/170FAN/Gas5 for 15 minutes per pound or 500g. For this weight, this equates to about 30 minutes. Check at 25 minutes that it is around 55C internal temperature for medium rare.
Chop the parsley, including the stalks. and crush or mince the garlic. Strip the leaves from the oregano stems and chop.
Combine all of the ingredients together to make the chimichurri sauce.
Allow the meat to rest for 15 minutes before carving into thick slices.