Heat oil in a pan and heat mustard until it begins to pop.
Add the garilc paste, lemongrass, onion and curry leaves and saute for 1 minute.
Add the onion and saute until the onion is cooked well.
Add the tomato and chilli pieces and saute for 1 minute.
Add the coconut milk and simmer for a few minutes.
Add salt to taste and serve.