In a bowl, season the fish fillet with the beaten eggwhite, half of the corn flour, soy sauce, salt and pepper. Mix well and let the fish marinate for 20 minutes.
In another bowl whisk the tomato sauce, vinegar, water, 2 teaspoons corn flour and fish sauce together.
Heat oil for deep frying in a wok until hot. Lightly coat the seasoned fish fillets with the leftover corn flour and deep fry until golden brown and crispy. Drain on paper towel. Arrange on serving plate and keep warm.
Heat a oil in a pan and saute the kaffir lime leaves with garlic, add onions and red chillies; stir fry for a few minutes. Pour in the tomato sauce mixture and add the sugar. While stirring constantly, cook until the sauce thickens and check seasoning.
Pour sauce over fish on the platter and serve with rice.