For the Halibut:
Season the halibut pieces with salt and white pepper. Put the potato flour or cornstarch, beaten egg, and breadcrumbs in three separate bowls. Dip the halibut pieces into the potato flour or cornstarch, then the egg, and coat in the breadcrumbs.
Place a wok over high heat and add all but 1 tbsp of the peanut oil. Heat the oil to 350degreesF, or until a piece of bread dropped in turns golden brown in 15 seconds and floats to the surface. Fry the breaded halibut pieces in the oil for 3-4 minutes or until golden brown, then remove with a slotted spoon and drain on paper towels.
For the Sauce:
Meanwhile, make the sauce. Heat a small wok or saucepan over medium heat and add the remaining 1 tbsp of peanut oil. Add the ginger and fry for a few seconds, then add the remaining ingredients and bring to a boil. Cook for 1 minute or until the sauce has thickened, then remove from the heat.
When the fish is cooked, season with dried chili flakes (if using), garnish with lemon wedges, if you like, and serve with the lemon dipping sauce.