Extra Crispy Haemul Pajeon (Seafood Scallion Pancake)

Name: Extra Crispy Haemul Pajeon (Seafood Scallion Pancake)
Prep time: 10 min
Cook time: 20 min
Total time: 30 min
Cuisine: Korean
Ingredients: 2 tbsp soy sauce; 1 tbsp rice vinegar (or white vinegar); 1 tbsp water; 2 tbsp chopped onion; 1 tbsp chopped red chili (optional); 2 bunches scallions; 1 cup (120 g) all-purpose flour; 2 tbsp cornstarch; 1 cup (240 ml) carbonated water (chilled); 1 tsp fish sauce; 5 tbsp oil (divided, and use more as needed); 1 1/2 cup (360 ml) mixed seafood (shrimp, squid, and mussel) (fresh or frozen (thaw first if frozen)); 2 eggs (slightly beaten); 2 fresh red chili (sliced, optional)