Heat the oil in a large saucepan and fry the onion over a medium heat for about 8 minutes, stirring until pale golden. Add the curry paste and stir-fry for about 1 minute.
Stir in the rice, then add the water, sweet potato, tomatoes and sultanas. Bring to the boil, then reduce the heat to a simmer. Cover the pan and leave on a low heat for 20–25 minutes, stirring occasionally, until the rice is just tender and the water has been absorbed. Add a little more water if the mixture becomes too dry before the rice is cooked.
Stir in the chickpeas and gently heat through. Stir in the coriander, season to taste and scatter with the cashew nuts before serving.