1. For the dipping sauce: In a small bowl, stir together the soy sauce, vinegar, gochugaru, sesame oil, sesame seeds and spring onions. Cover and set aside. Store in the refrigerator if not using immediately.
2. 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.
3. 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-towellined 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.