Heat the oil in a wok or saucepan over a medium heat. Add the mustard seeds and as soon as they begin to 'pop' (after a few seconds), add the onion, garlic and chillies. Cook for about 5 minutes until the onion is soft and translucent.
Add the remaining spices and the beetroot. Fry for a further 1–2 minutes, then add the tomatoes, measured water and a pinch of salt. Simmer for 15–20 minutes, stirring occasionally, until the beetroot is tender.
Stir in the coconut milk and simmer for a further 1–2 minutes until the sauce has thickened. Stir in the lime juice and check the seasoning. Garnish with chopped coriander and serve immediately.