Place the lamb In a large resealable bag along with all of the ingredients for the marinade. Shake the bag so that everything is well mixed and then place the bag in the fridge to marinate for a minimum fo 6 hours but ideally overnight.
Preheat the oven to 200C (390F). Heat a grill pan to medium high heat and sear the lamb for 4-5 minutes per side until nicely golden brown in colour. Once both sides are seared place the pan in the oven and roast the lamb for 25 minutes or until it reaches 55-60C (130-140F) for rare or 60-65C (140-145F) for medium
While the lamb is cooking make the chimichurri. In a food processor combine all of the ingredients for the chimichurri and pulse until the herbs are broken into small pieces. Taste and adjust the seasoning as needed.
Once the lamb is done roasting, remove from the oven, place on a cutting board and cover with tin foil and leave to rest for 5 minutes.
After 5 minutes cut the lamb into slices, arrange them on a serving platter and drizzle the chimichurri overtop or alternatively serve the chimichurri on a bowl on the side.