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 55◦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.