- Marinate the fish: Clean and pat-dry the fish fillets. Cut into 1.5 to 2-inch pieces. Transfer to a bowl and add the kosher salt, low sodium light soy sauce, Shao Xing rice wine, and sesame oil. Use a spoon to gently mix and coat the fish pieces with the marinade. Set aside to marinate for 10 minutes while you prepare the rest of the ingredients.
- Make the sauce: Whisk together the potato starch, white sugar, kosher salt, black pepper, low sodium light soy sauce, Shao Xing rice wine, dark soy sauce, oyster sauce, sesame oil, and water in a small measuring cup (or bowl) until combined well.
- Prepare the fresh/dry ingredients: Chop/slice the spring onion white and light green parts, garlic, ginger, red chilies, yellow onion, and red bell pepper as indicated in the ingredients section. Rinse the black beans and smash with the flat side of your knife. Remove the stems of the dried red chilies and snip into ½-inch pieces. (Shake out and discard some or all of the seeds for a milder dish if desired.)
- Coat the fish in starch: Add 2.5 tablespoons each of corn starch and potato starch to a large Ziploc bag. Mix with a spoon until evenly combined. Working with one piece at a time, shake off the excess marinade and add to the bag. Turn to coat with the starch mixture, then shake off the excess starch and transfer to a clean plate. Repeat with the remaining fish pieces until all are coated in the starch mixture.
To Fry the Fish:
- Fry the fish: Fill a small pot with peanut oil until it is 1/3 of the way full. Heat over high heat until the oil reaches a temperature of 175°C/350°F. Then turn the heat down to low. Working in batches of 5-6 pieces at a time, add and fry the fish for 2-3 minutes, turning occasionally, until lightly golden and crispy. Using a metal slotted spoon and transfer to a paper towel lined plate to drain. Repeat until all of the remaining fish pieces are fried.
For the Wok Fried Fish in Black Bean Sauce:
- Stir fry the aromatics: Heat 3 tablespoons of peanut oil in a large wok over high heat. Once hot, add the spring onion white and light green parts, garlic, ginger, and fresh red chilies. Stir-fry for 30 seconds until fragrant.
- Add the black beans and dried red chilies. Add the fermented black beans and dried red chilies. Stir-fry for 20-25 seconds, taking care to not let the dried chilies burn.
- Add the onion and bell pepper. Add the yellow onion and red bell pepper. Toss for 10 seconds to combine.
- Add the fish and sauce: Give the sauce a quick stir with a spoon to loosen up the starch that will have settled at the bottom. Add the fried fish pieces into the wok and pour the sauce over everything. Stir-fry using a scooping motion, taking care to not break any of the fish pieces, until everything is coated well in the sauce – about 30-45 seconds. Switch off the heat.
- To Serve: Transfer to a serving plate and garnish with chopped spring onion dark green parts (if using). Serve immediately with warm steamed rice and a side of stir-fried Asian greens if desired.