Clean and wash the tilapia, remove grills, innards and scales. Rinse to remove blood and remaining scales.
Cut a slit on each side of the fish and rub with rock salt.
Fry the tilapia until the skin is crispy and set aside.
In a medium size wok or frying pan, heat some oil and saute onions for about 2 minutes or until the onions are soft. Remove from the pan and set aside.
Pour in water, calamansi juice, soy sauce, MSG, sugar and ground pepper then let boil, add also some of the sauteed onions and simmer for a few minutes.
Then add in the fried tilapia and simmer for 1 minute to soften the fish and add the onions.
Simmer for 1 minute more then remove from heat and serve hot.