Heat the oil in a large, heavy-based frying pan and cook the onion and lamb over a high heat, stirring frequently, for 5 minutes until the lamb is browned all over and the onion softened.
Add the chilli, if using, and cook, stirring, for 1 minute. Stir in the curry paste and cook, stirring, for a further 2 minutes. Add the tomatoes, stock and potatoes and bring to the boil. Reduce the heat, cover and simmer for 10 minutes, then remove the lid and cook for a further 10 minutes until the lamb is cooked through and the potatoes are tender.
Remove from the heat, then scatter over the coriander and spoon in the yogurt, ready to stir in and serve.