Wash and drain the fish. Cut into 2-inch cubes and apply 1 tablespoon of lemon juice.
Puree onion, tomatoes, coconut, ginger, and garlic to a fine paste.
Heat oil in a medium pan. Add mustard seeds and once they start to splutter carefully add the onion and tomato puree.
Add turmeric and red chili powder and cook on medium heat for 5 minutes until the oil starts to separate stirring occasionally.
Drain any water/lemon juice from the fish and add the fish to the pan.
Add salt, water and gently mix together the sauce and the fish. Cook covered on medium heat for 5 mins. For thicker fish cook up to 10 mins. Add more water if needed to bring the gravy to your desired consistency.