Pour water in a cooking pot. Let boil.
Add the tomato and onion. Cover and continue to cook in medium heat for 12 to 15 minutes. You can add more water if needed.
Remove the cover. Mash the tomato and onion to extract the flavor.
Add the fish. Cover and cook in low to medium heat for 15 minutes.
Pour-in the fish sauce. Stir and cook for 3 minutes or until the liquid reduces to your desired amount.
Transfer to a serving bowl. Serve.
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