Wash pompano inside and out and pat dry. Cut into half, if desired, and season with salt to taste and pepper to taste.
In a wide pan over medium heat, heat oil. Add onions, garlic, and ginger and cook, stirring regularly, until softened.
Add tomatoes and cook, mashing with the back of a spoon, until they begin to soften and release juices.
Add fish sauce and continue to cook, stirring occasionally, for about 1 minute.
Add water and bring to a boil for about 2 to 3 minutes or until tomatoes are completely softened.
Add tamarind powder and stir until dissolved.
Add fish in a single layer. Lower heat and cover. Continue to cook for about 10 to 15 minutes or cooked through and liquid is reduced.
Season with salt and pepper to taste. Serve hot.