Prepare the marinade by pounding the garlic, ginger and chilli with the salt in a mortar with a pestle to form a coarse paste. Add the coriander and parsley and continue to pound to a paste. Beat in the cumin, ground coriander, olive oil, honey and lemon juice.
Cut small incisions in the shoulder of lamb with a sharp knife and rub the marinade all over the meat, making sure it goes into the incisions. Cover and marinate in the refrigerator for at least 6 hours or overnight.
Place the shoulder of lamb in the base of a tagine and pour over the olive oil. Scatter the onion wedges around lamb and pour over the measurement water. Cover and place in a preheated oven, 180°C (350°F), Gas Mark 4, for about 1 hour 20 minutes.
Melt the butter in a heavy-based frying pan over a medium heat, add the quinces and stir to coat well, then sauté for 3–4 minutes, depending on size, until golden brown.
Remove the tagine from the oven. Arrange the quinces around the lamb and drizzle the honey over the quinces. Re-cover, return to the oven and cook for 15 minutes, then remove the lid and cook for a further 10 minutes. Scatter the chopped parsley over the top of the lamb and serve with lemon wedges.