Bring the lamb up to room temperature as this will help to ensure that the lamb cooks more evenly throughout.
Preheat the oven to 200 C / Gas 6.
Line a baking tray or roasting tin with aluminium foil. Rub the lamb all over with 4 tablespoons olive oil then season generously and rub with rosemary and garlic.
Heat the remaining oil in a large frying pan over high heat. Place lamb in frying pan when the oil is very hot. Brown well on all sides, about 2 minutes each side. Remove from heat and immediately transfer lamb to prepared tin. Stand the racks up on the meat end facing each other with the bones interlocked.
Roast in the preheated oven for 20 to 25 minutes for medium, cook in increments of 10 minutes for a more well done lamb. Remove from the oven, transfer to a plate and let rest 10 minutes before slicing.
For the best flavours, rub the lamb in oil, herbs and garlic several hours prior to cooking.