Preheat the oven to 180°C (160° fan-forced). Line a large roasting dish with baking paper.
In a small bowl, place half the oil, chopped coriander, garlic and tandoori paste. Season and stir to combine.
Spread onions onto roasting dish, season and toss to coat. Make small incisions in lamb with a sharp knife, spoon marinade over lamb and rub well to coat. Place lamb on top of onions, pour 1/3 cup water into base of dish, cover with foil and roast for 2 hours. Remove foil, spoon juices over lamb and add pumpkin to dish. Cook for a further 30-40 minutes, or until lamb is cooked to your liking and vegetables are tender. Remove lamb from dish and set aside covered with foil for 15 minutes to rest.
Place cooked onions and pumpkin, chickpeas, baby spinach and green beans in a large bowl. Drizzle with remaining oil, season and toss to coat.
Shred or slice lamb and serve with pumpkin salad, rice, yoghurt, lime wedges, extra coriander and any remaining pan juices.
Ask your butcher to bone out and truss the lamb leg for you.
Switch lamb leg for lamb shoulder.
Use leftover leg of lamb in salads, tacos, or fritters.