Mix the sauce in a bowl by adding the sugar, hot water, soy sauce, oyster sauce, wine, sesame oil and white pepper.
Use a paper towel to pat the fish dry on both sides, and rub the inside of the fish cavity with a large pinch of salt.
Heat a carbon steel wok or large frying pan over medium-high heat until just smoking and add the oil to coat. Carefully lay the fish down and let it fry for 7 minutes on the first side—don’t move the fish until it forms a crust and releases easily (around the 5-minute mark). Pick up the pan and swirl the oil around the fish. If using fillets, allow it to cook for 2-3 minutes until a crust forms.
After 7 minutes, flip the fish over using a fish spatula (or a couple spatulas!) Once again, don't move it, and this time, cover the pan. After 5 minutes, check on the fish. By then, it should easily release from the pan and slide around. Continue to cook for another 2 to 3 minutes until the inside cavity of the fish is opaque. You can use an instant-read thermometer to check that the internal temperature near the bone is 140°F/60°C. Cover again if it needs more time to cook. If using fillets, all you need to do is sear the other side for 1-2 minutes.
Carefully transfer the fish to a plate. It will continue cooking and should reach 145°F/63°C as it rests. Cooking times will vary depending upon the size of your fish, so using a thermometer is more precise.
Over medium heat in the same pan, add the garlic and white portions of the scallions. Cook for about 10 seconds and add the sauce mixture. Stir and bring to a simmer.
Reduce the sauce for about 1 minute, stirring constantly, and add the green parts of the scallion and the butter. After the butter has melted and the sauce starts to simmer, turn off the heat and pour the sauce over the fish. Serve.