Oil a frying pan with 3 tbsp oil. Once the oil is hot (oil starts sputtering), lower whole fish into pan and allow it to fry for 2-3 minutes under medium heat. Flip the fish over and allow it to fry for another 2-3 minutes. Insert a fork into the fish to see if it is cooked; if the fork can be easily inserted, the fish is cooked, if not, continue frying. Remove from pan.
In the same pan, add onions, tomato and ginger and fry until fragrant.
Mix cornflour and water in a small bowl.
Put fish back into the pan. Drizzle the cornflour mixture over the fish and allow the water to start boiling. Once water boils, turn off heat. Serve.