To make tandoori paste, place onion and garlic in a small food processor and process until finely chopped. Add ginger, curry powder, tomato paste, lemon juice, chilli and 1/2 tsp salt and process until a thick paste forms.
Make cuts in the lamb using the tip of a small knife. Place in a large snap lock bag. Spread paste all over lamb to coat, and rub in well. Refrigerate overnight.
Remove lamb from fridge and allow to come to room temperature (at least 30 minutes).
Preheat oven to 170°C. Place lamb and marinade in a roasting pan lined with baking paper. Add stock to pan. Cover lamb with baking paper and then cover the pan tightly with foil. Bake for 2 hours or until lamb is very tender.
Remove pan from oven, uncover, discarding foil and paper. Return pan to oven and cook for a further 30 minutes or until lamb is golden. Scatter with coriander and serve with yoghurt and lemon wedges.