Slash the lamb a few times all over. Mix a few thyme sprigs, the tagine spices (from all 3 sachets) and 1 tbsp oil, then rub all over the meat. Sit it on top of the onions and garlic in a large roasting tin, then (if time) marinate in the fridge overnight. If not, no problem.
Remove the lamb from the fridge 30 minutes before starting to cook. Preheat the oven to 200°C, gas mark 6. Roast the lamb for 30 minutes until golden, then tip in 500ml boiling water from the kettle. Lower the oven to 160°C, gas mark 3. Cover tightly with foil and bake for 2½ hours.
Meanwhile, heat a large frying pan. Fry the aubergines in 1 tbsp oil for 10-15 minutes. Once soft and golden, stir in a little more thyme plus the tomato paste. Leave to caramelise for 1 minute. Mix in the chick peas, harissa and honey then set aside. Stir the feta into the yogurt, season and chill.
Spoon off the excess fat from the meat juices. Add the chick pea mix to the roasting tin, re-cover and bake for 2 hours until the lamb melts from the bone and the aubergines collapse into the sauce. Serve with more thyme leaves, the feta yogurt, plus a green salad and flatbreads.