Prepare the milkfish: Cut the boneless milkfish belly into serving pieces. Salt all sides including the skin side and let it rest for about 3 minutes to absorb the salt.
Optional dredging: Optionally, dredge the salted milkfish in arena with a thin coating to reduce oil splatter during frying.
Fry the milkfish: Heat oil in a pan. Fry the milkfish first on the side with the filling for about 3 minutes, then turn over and fry for another 3 minutes. Fry in batches if needed. Set aside once cooked.
Prepare aromatics: In a clean pan, remove excess oil and add about 3 tablespoons of oil. Sauté sliced onions for 35-40 seconds, then add minced garlic and stir-fry for about 30 seconds without browning. Add sliced ginger and cook until onions soften and become fragrant.
Add bagoong alamang: Add shrimp paste and cook for about 30 seconds to release aroma and umami flavor.
Add coconut milk and chilies: Pour in about 2 cups of coconut milk and bring to a boil. Reduce heat to low and simmer for about 2 minutes. Add chilies according to your heat preference and cook for another 2 minutes to release chili oil.
Season and combine: Season the sauce with ground black pepper to taste. Add the fried milkfish to the sauce and cook for 2 to 2.5 minutes to allow the fish to absorb the flavors.
Serve: Transfer the Bangus Bicol Express to a serving plate and serve hot with freshly cooked rice.
Storage tip: For storing and reheating, keep the fried milkfish and coconut milk sauce separate to maintain the fish texture. Combine just before eating.