With a knife, score fish by making 4 slits on both sides. Evenly coat 4 tbsp turmeric powder and 4 tbsp cornstarch on both sides of the fish
In a pot, heat up enough vegetable oil for deep-frying the whole fish. Deep-fry the fish on high heat for 10 minutes, until crispy. Drain excess oil on a paper towel-lined plate and set aside.
In a pan on medium-high heat, heat up 2 tbsp vegetable oil and sauté shallots, garlic and ginger for 1 minute,until fragrant.
In the same pan, mix in 5 tbsp kecap manis, 4 tbsp light soy sauce, 1 tbsp dark soy sauce, 200 ml water and thickener together and let it reduce until sauce thickens.
Season with ground white pepper to taste
Transfer dish to a large plate and drizzle thickened sauce over fried fish. Garnish with finely julienned spring onion and large red chili and serve hot.