Crush the peppercorns with the cumin and cardamom seeds in a pestle and mortar or an electric grinder. Set aside.
Heat the oil in a large flameproof casserole dish, add the onion and fry gently for 5 minutes or until softened. Add the garlic, ginger and chilli and fry for a further 3 minutes, then add the crushed spices and the cinnamon stick and turmeric. Fry gently for 30 seconds, stirring constantly.
Add the lamb and stir to coat with the spices. Fry gently for about 4 minutes or until the meat is browned all over. Gradually pour in the stock, stirring well, and bring to the boil. Reduce the heat, cover and cook gently for 1 hour or until the lamb is almost cooked and tender.
Meanwhile, rinse and drain the lentils, then place them in a saucepan and cover with cold water. Bring to the boil. Reduce the heat and simmer, uncovered, for 15 minutes. Drain.
Add the lentils and tomatoes to the curry and cook for about 15–20 minutes or until the lamb and lentils are both tender. Stir in the lemon juice and fresh coriander and season with pepper. Serve hot.