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Take the lamb out of the fridge while you chop the onions, squash and celeriac. Heat oven to 200C/180C fan/gas 6. Trim any excess fat off the leg of lamb, then cut a few slashes into the meat. Rub ½ tbsp oil and 1 tbsp ras el hanout over the lamb and season with salt and pepper. Put the onion, celeriac, butternut squash into a large roasting tin with the garlic. Toss with the remaining ras el hanout, remaining oil and some salt and pepper. Nestle the lamb into the tin and put in the oven to roast for 40 mins.
Take the lamb out of the oven and leave to rest. Put the veg back in the oven for 20 mins. Meanwhile, blitz the coriander, cumin seeds, lemon zest, lemon juice and green chilli together in a mini food processor until finely chopped and vivid green.
Carve the lamb, put on a platter, then pile on the veg. Sprinkle over some of the coriander mixture before taking the platter to the table for everyone to help themselves.