Put the oil in a deep, wide saucepan on a medium heat, and add the cumin and mustard seeds. As soon as they start to pop, add the curry leaves and sliced onion, and cook, stirring, over a high heat for three minutes. Add the ginger and green chillies, cook, still stirring, for two minutes more, then turn down the heat to medium, add the tomatoes and cook, stirring occasionally, for five minutes.
Stir in the salt, turmeric, sambar powder and sugar, then pour in 500ml just-boiled water and leave to bubble away gently for 10 minutes.
Take off the heat, sprinkle the chopped coriander over the top and leave to rest for 10 minutes. Serve with rice or bread.
