Heat oil in a pan and temper with mustard seeds. After it stops spluttering, add the garlic and saute till they changes colour.
Add the onion, curry leaves and green chilies. Fry till the onions turns translucent. Then add all the dry spices and mix well, Add the tomatoes and fry till it is mashed.
Now add the thin coconut milk and the fish. Cover and simmer for 4-5 minutes. Lastly add the thick coconut milk and bring it to a boil. Switch off the flame and add the coriander leaves. Serve with plain steamed rice.