Heat the oil in a kadhai or a heavy bottomed pan. Add the mustard seeds and let them splutter. Add the curry leaves and the onion

Once the onion has softened, it need not turn brown, add the tomatoes, turmeric powder, red chilli powder and the green chillies

Add salt and saute for a couple of minutes, till the tomatoes soften. Add the water and let it come to a boil.

Add the vermicelli and keep stirring, so as to not form any lumps. Cook over low heat while stirring gently, till all the water is absorbed and the vermicelli is cooked through