Prepare the milkfish: Cut the boneless milkfish belly into serving pieces. Salt both sides of the milkfish and rub the salt in. Let it rest for about 3 minutes.
Coat the milkfish: Optionally, lightly coat the milkfish pieces with all-purpose flour to reduce oil splatter during frying.
Fry the milkfish: Heat cooking oil for about 2 minutes. Fry the milkfish starting with the side that has the meat first. Fry in batches if necessary. Drain excess oil and set aside.
Prepare aromatics: While frying, slice and chop the long chili, Thai chili, ginger, onion, and garlic.
Sauté aromatics: In about 3 tablespoons of the frying oil, sauté onion first until slightly soft, then add ginger and garlic. Stir fry for 30 to 40 seconds.
Add bagoong alamang: Add the shrimp paste to the sautéed aromatics and cook for about 20 to 25 seconds.
Add coconut milk and chili: Pour in about 2 cups of coconut milk and bring to a boil. Reduce heat to simmer and add sliced long chili and Thai chili peppers. Simmer for 2 to 3 minutes until the sauce starts to get oily.
Season and combine: Season the sauce to taste. Add the fried milkfish pieces into the sauce and cook on medium heat for 1 to 2 and a half minutes, covered. Do not overcook to avoid softening the fish too much.
Serve: Serve immediately. For meal prep or packed meals, keep the fried milkfish and sauce separate until ready to eat to maintain texture.