In a pan, heat oil add mustard seeds. Once it starts spluttering, add curry leaves and whole red chilly add chopped onions and fry onions in it till golden brown.
Add ginger garlic paste and fry it for few seconds.
Add tomatoes and salt , cook it for few minutes till tomatoes are cooked.
Then add fish curry masala and fry for 2 minutes.
Add fish pieces and bhuno them by turning occasionally on medium flame.
Now add water and stir.
Cover and simmer on low flame.
Add more water if required and cook till it is tender.
Garnish with chopped coriander.