Clean the shrimps and marinate them in turmeric powder, salt and a spoonful or two of yoghurt. Chop the onion, tomatoes and the garlic. In a pan, heat the mustard oil and fry the chopped onions and garlic. When slightly golden brown put in the tomatoes along with the spices and cook for about a minute before adding the marinated shrimps. Add a little bit of salt and another spoonful or two of yoghurt. Cover with a lid and cook for about ten minutes or so on low flame. The shrimps should be done by now. Throw in the chopped chillies and the coriander.