Start this the night before you want to cook. Place all of the lamb ingredients apart from the meat itself into a blender and blitz to a smooth paste to make a marinade.
With a knife, make some little incisions into the lamb shoulder to help the marinade get right into the meat. Rub and massage the marinade into the shoulder like it’s date night, until it’s completely covered.
Transfer the lamb to a roasting tin, cover with foil and place it in the fridge overnight (or for a minimum of six hours).
Put the couscous into a container big enough to allow it to double in size and pour in 400ml/about one-and-a-half cups of cold water. Cover the bowl and transfer it to the fridge. Leave this overnight, too.
Remove the meat from the fridge 30 minutes before you intend to start cooking so that it can come up to room temperature, and preheat the oven to 190 °C/170°C fan/375F/Gas 5.
When you’re ready to cook, roast the lamb, still covered with the foil, for four hours, until it is charred a little on the outside and the meat is tender and pulls apart.
Drain the couscous through a fine sieve, so you don’t lose any of it. Mix all of the other couscous ingredients into it. Season with salt and pepper to taste and leave on the side to come up to room temperature.
Either serve your massive hunk of delicious lamb in the tin as it comes, or transfer it to a wooden board and pour all of the sauce that is left in the bottom of the roasting tin into a little jug.
Serve with warm flatbreads and just let people dig and tear into this huge, sharing-lamb deliciousness, the couscous and the sauce served alongside.