Combine oil and sumac in a bowl, coat lamb with mixture. Season with a little salt.
Heat a large frying pan over high heat until very hot. Sear the lamb on both sides then transfer to a gastronome. Insert a probe, roast to internal doneness of 56°C.
Chermoula: Combine oil, lemon rind, juice, garlic, chilli flakes, cumin, paprika, cinnamon and salt. Add coriander and parsley . stir to combine. Season to taste and add a little more lemon juice if necessary.
Remove lamb from the oven and transfer to a board. Cover with foil and rest for 10 minutes. Thickly slice the lamb and serve with the chermoula and a tomato salad.
Notes
If time permits marinated lamb for up to 2 hours.
Substitute butterflied shoulder or boned and rolled shoulder.
