Set the fish on a cutting board belly up. With the blunt end of a cleaver, hit the fish a few times to crack the bones along the stomach. This makes it easier to serve the cooked fish right side up instead of on its side.
Score the sides of the fish deeply, right down to the bone, at 1½-inch intervals. This helps the fish cook more evenly and quickly.
Coat the fish evenly with cornstarch or water chestnut powder, making certain that the interior of the scoring cuts is coated as well. Shake the fish to remove any excess powder.
Heat the 4 cups of oil in a wok until moderately hot (360 degrees). Slip the fish in head first. (The front end of the fish is the thickest part and needs the most cooking.)
Scoop the hot oil over the fish to help it brown evenly.
As soon as it is lightly browned (after 5 to 8 minutes), remove the fish with a strainer, turn off the heat, and drain all but 2 tablespoons of the oil. (The excess oil may be strained and reserved for use with other fish preparations.)
Over high heat, stir-fry the peppers, garlic, and ginger to flavor the oil.
Add the sauce ingredients. As they come to a boil, the sauce will thicken.
Slide the fish into the sauce, reduce the heat to a simmer, and baste the fish with the sauce regularly while the fish is braising. Test for doneness by pushing one of the scored cuts with a chopstick to see if the fish is opaque all the way to the bone.
Transfer the fish and the sauce to a serving plate and sprinkle it with the chopped scallions.
Best served immediately, but can be reheated slowly in braising liquid just before serving.