First, start by cleaning the bangus. Remove the innards and gently wash the fish, keeping the scales on if you prefer. Cut the fish crosswise into 2-inch thick slices.
Crush 4 cloves of garlic and slice one thumb-sized piece of ginger into strips. Cut one large eggplant into 1-inch thick wedges. Slice one onion thinly.
Take your cooking pot or kawali and layer the sliced onions, crushed garlic, and ginger strips at the bottom.
Place the bangus pieces on top of these aromatics. Try not to stack or overlap the fish pieces. Arrange the eggplant wedges and finger chilies around the fish.
Pour ½ cup vinegar and 1 cup water over everything. Add 1½ teaspoons salt and 1 teaspoon whole peppercorns. Do not stir.
Turn the heat to high and let it come to a boil. Once boiling, lower the heat and cover the pot. Let it simmer gently for 15 minutes. Remember not to stir, as this will break the fish.
After 15 minutes, check if the fish is cooked through. The flesh should be opaque and flaky.
Here's the special step that makes this recipe perfect: drizzle 1 teaspoon of cooking oil over the top. Instead of stirring, gently shake the pot to spread the oil.
Let everything simmer for 2-3 more minutes, then turn off the heat. Let it rest for 5 minutes before serving.
Serve hot with rice and fish sauce on the side. This dish tastes even better the next day, after the flavors have had time to develop.