Preheat oven to 240 degrees Celsius.
Thoroughly clean the milkfish and pat dry. Make a 6 - inch slit at the back of the milkfish to cut it open, Wash again and pat dry.
Squeeze the calamansi inside the milkfish. Drizzle the soy sauce and season it with ground black pepper.
In a bowl, mix together garlic, onion, ginger, tomatoes and onion spring. Reserve 2 tablespoons for the dipping sauce. Stuff the milkfish with the garlic mixture.
Wrap the milkfish with an aluminum foil brushed with olive oil. Baked for 20 minutes. Remove the aluminum foil and bake for another 10 minutes until the skin is golden brown. Serve with soy sauce dipping sauce.
For the soy sauce dipping sauce, combine the reserved stuffing, calamansi juice and soy sauce. Add chili if desired.