For the dipping sauce: In a small bowl, stir together the soy sauce, vinegar, gochugaru, sesame oil, sesame seeds and scallions. Cover and set aside. Store in the refrigerator if not using immediately.
For the fish: Put the flour and beaten egg into separate wide, shallow bowls. Cut each fish fillet crosswise at a slight angle into four or five pieces and sprinkle lightly with salt and pepper. Set aside for about 5 minutes, so the seasonings soak in a bit. Lightly dredge the fish in the flour, tapping off any excess.
In a large nonstick skillet, heat 2 tablespoons oil over medium heat. Working in batches, coat the fish in the egg, letting any excess drip into the bowl, and then place into the skillet. If desired, press a chile slice and some chives onto each piece. Cook until golden, flipping halfway through, about 4 minutes total. Transfer to a wire rack or paper-towel¿lined plate to drain. Repeat with the remaining fish, adding more oil to the skillet as needed.
Transfer the fish to a platter and serve immediately with the dipping sauce.