Heat oil in a pan, saute the onion until softened. Then, add the garlic, ginger and cayenne pepper. Mix.
Now, pour in the coconut milk and water. Stir until boil.
Then, add the vinegar, salt, and green chilies. Cover and simmer for 2 minutes.
After 2 minutes, add the fish. Cover and simmer for 5 minutes.
After 5 minutes, add the spinach leaves and simmer again for the last 5 minutes.
Transfer into a serving bowl.
Serve hot and enjoy!