To make the chermoula, in a blender, combine the oil, lemon rind, 2 tbs lemon juice, garlic, chilli flakes, cumin, paprika, 1 tsp sea salt flakes and 1/2 tsp pepper and blend until smooth. Add the parsley and coriander and blend until herbs are chopped but not pureed. Season with salt and pepper. Add a little more lemon juice, if necessary.
Prepare a covered barbecue for indirect cooking. For a gas barbecue, heat one burner on medium-high and leave other burners off. For a charcoal barbecue, build fire on one side of charcoal grate, leaving other side empty, and let burn until coals are just covered with white ash. Do not spread out coals. The temperature inside the barbecue should be about 200°C.
Brush lamb with 1 tbs oil and season with salt and pepper. Cook lamb over the lit side of the barbecue, rotating as needed, for 10-15 mins or until lamb is brown all over. Transfer lamb to the unlit side of barbecue. Cook, covered, for 1 hour or until an instant-read meat thermometer registers 60°C when inserted into the centre of the lamb. Transfer to a plate. Brush all over with 1/3 cup chermoula. Cover loosely with foil and set aside for 20 mins to rest.
Meanwhile, coat the combined capsicums with remaining oil. Cook over lit side of barbecue, turning occasionally, for 15 mins or until charred all over. Transfer capsicums to a heatproof bowl and cover with plastic wrap. Rest for 5 mins. Remove seeds, stems and skins from capsicums and discard. Slice flesh. In a bowl, toss the capsicum with 1 tbs chermoula. Season with salt and pepper.
Transfer lamb to a serving platter. Thickly slice. Serve with capsicum and remaining chermoula. Sprinkle with extra coriander.