Heat the oil in a kadhai or wok and fry the cumin and mustard seeds over a medium heat until they splutter.
Add the tomato and stir until coated with the seeds, then stir in the ground spices, salt and sugar.
Add 1 cup (250 ml) water, then reduce the heat and simmer for a few minutes – not too long, or the tomato will break down. Sprinkle with the fresh coriander to serve.