For the lamb, preheat the oven to 220°C and place the onions and garlic in a large deep roasting tray. Place the lamb shoulder on top, scattering with origanum and seasoning with salt and pepper all over. Add the wine and water to the tray, then cover with a lid (or with foil). Roast for 30 minutes, then turn down the heat to 120°C and roast for another 6–7 hours until very tender and falling from the bone.
About 30 minutes before the lamb is ready, place the couscous and boiling water in a bowl and season with salt. Cover with a lid or a plate and leave to steam for about 5 minutes. Fluff up the couscous with a fork, then add the mint and parsley, lemon juice and rind and olive oil. Stir well and add more salt if needed. Cover and set aside until ready to serve.
Briefly cook the peas in boiling water and drain, then stir through the butter and season with salt and pepper. When the lamb is ready and completely fall-apart tender, transfer it with most of the pan juices (if the pan juices are too much, pour into a small jug and serve on the side) to a large serving platter along with the peas, scatter with a few whole mint leaves, and serve with the couscous on the side.