Prick the lamb leg all over using a skewer.
Combine the marinade ingredients and coat the lamb thickly.
Cover the lamb and refrigerate to marinate for two hours.
Preheat the oven to 180°C.
Place lamb in a roasting tin, cover loosely with foil, and roast for 120 minutes.
Remove the foil and roast for a further 8-10 minutes.
Remove from oven, recover with foil, and rest for 15 minutes.
Whip the raita yoghurt and fold in onions, tomatoes, chilli, and salt.
Heat oil in a pan and fry mustard seeds, curry leaves, and optional peanuts.
Cool the spiced oil mixture before stirring into the yoghurt.
Simmer the roasting tin juices with 75-100ml boiling water for three minutes to make gravy.
Carve the meat and serve topped with the spicy gravy and chilled raita.