Preheat oven 200C. Brush the olive oil over the lamb and rub sumac into the oil. Season with a little sea salt.
Heat a large frying pan over a high heat until it is very hot. Sear the lamb on both sides then transfer to a roasting dish. Roast for 40-45 minutes.
To make the chermoula, place all the ingredients except the olive oil in a food processor and pulse. With the motor running, drizzle in the oil to form a thick paste.
Combine the tomatoes with the basil, vinegar and olive oil. Season with sea salt and freshly ground black pepper and toss to combine.
Remove lamb from oven and transfer to a warm plate. Cover with foil and rest for 10 minutes. Thickly slice the lamb and serve with the chermoula and heirloom tomato salad.
