Roast the cumin seeds, coriander seeds and peppercorns in a dry frying pan over a medium heat for about 5 minutes until they are toasted and fragrant.
Crush the toasted seeds in a pestle and mortar until finely ground and add them to a food processor.
Peel the garlic and ginger and add to the food processor.
Add the roasted peppers, tomato puree, oil, red chilli and greek yoghurt to the food processor.
Add the spices to the food processor (the smoked paprika, turmeric, garam masala & salt).
Blitz in the food processor until you have a paste.
Prepare the Leg of Lamb
Slice the onions and arrange in a layer in the bottom of an an oven-proof dish.
Score the leg of lamb with a sharp knife in a criss-cross pattern. Rub the curry paste all over the surface of the lamb coating thoroughly.
Place the leg of lamb on top of the layer of onions, cover the dish in foil and leave to marinade overnight in the fridge. (You can skip this step if you don't have the time and just cook straight away).
Cooking the lamb
Preheat the oven to 200C (390F).
When it's up to temperature, place the foil covered lamb dish into the oven and turn the temperature down to 140C (284F).
Roast for 6 hours until the meat is tender and falls off the bone. Cooking times will vary depending on the size of your leg of lamb (approximately 1 hour for every 500g).
To Serve
Remove from the oven and sprinkle over the chopped pistachios, pomegranate seeds and chopped fresh mint just before serving.