For two large sirloin steaks, marinate in bowl containing 1 grated red pinion, juice of 4 limes, 2tsp sumac, 1tsp of salt & 1tbs ground black pepper. Grate the zest of the lime skins in too. Soak & turn in fridge for 24 hrs. Take out 1 hr before cooking, pick off any bits of marinade and pat dry (otherwise it will stew & not sear). Cook very hot in a olive oiled skillet for 2mins ea side - that’ll be rare / medium-rare for a 15mm thickness. Make sure it’s hot enough to really caramelise the outside.