Heat the oil in a large, lidded, non-stick saucepan over a medium heat. Add the lamb, onions and garlic and fry for 5-6 minutes until lightly browned, stirring regularly.
Add the curry paste and stir well to coat the lamb and onions. Continue to cook for 1-2 minutes, stirring well, then add the tomatoes, lentils, 500ml/18fl oz water and salt. Bring the mixture to the boil. Reduce the heat until the mixture is simmering, then half-cover the pan with the lid and simmer very gently for 45-50 minutes, stirring regularly, until the lamb is very tender. (Add a little extra water if the curry is beginning to look dry before the lamb is tender.)
Season with salt and pepper, stir in the kale and simmer for a further 2-3 minutes, or until tender.
Serve in bowls with steamed rice or warm naan bread.