Step 1

For the spice mixture, toast the cumin, coriander and fennel seeds in a dry frying pan until fragrant. Cool slightly, then pound with a pestle and mortar or spice grinder until finely ground. Reserve 2 tsp of the mixture, then add the rest to a blender along with the chopped onion, garlic and ginger. Pulse until a paste forms. Add the turmeric, cinnamon, ground ginger, cloves, olive oil and lemon juice, then season well.

Step 2

Pierce the lamb all over with the tip of a sharp knife and put in a large container or sealable food bag. Rub the marinade into the meat on all sides, then chill for at least 12 hours or up to 48 hours.

Step 3

Heat the oven to 160C/fan 140C/gas Arrange the onion slices in the bottom of a roasting tin and put the lamb on top. Pour 500ml of water around the lamb in the tin and cover with foil. Roast for 4 hours, checking the water halfway through and topping it up if it is drying out.

Step 4

Remove the foil and turn up the oven to 190C/fan 170C/gas 5 and roast for 30 minutes to crisp the skin and turn the crust deep golden brown.

Step 5

To make the harissa, whizz the ingredients in a food processor with the reserved spice mixture. Add a pinch of sugar to balance the saltiness in the pickled chillies or some seasoning, if you like. Shred the lamb with two forks, then serve alongside tahini yogurt, flatbreads or pitta, herbs and the pickled chilli harissa.