1. Scale the tilapia, remove its gills and guts, rinse and blot dry, then cut a few slits on the body of the fish.
2. Thinly slice onion; dice chilli pepper; smash lemongrass with a blade then cut into sections; smash garlic cloves, set aside.
3. Mix oyster sauce, soy sauce, rice wine and white pepper to make the sauce, set aside.
4. Prepare a piece of aluminium foil, spread 1/3 of the ingredients, including onion, lemongrass, Thai sweet basil, Kaffir lime leaves, scallion, coriander, chilli pepper and garlic over the foil, then place the tilapia on top, stuff a little ingredients inside its stomach, pour sauce over the fish and inside its stomach, scatter the rest of the ingredients over the fish, then wrap it in a foil packet.
5. Preheat oven to 200℃, bake the fish for 30 mins.
6. Remove the fish from the oven, brush aside the ingredients, season with pepper and a little lemon juice and serve.