Prepare the batter: in a large bowl whisk together the All-Purpose Flour and Rice Flour with a pinch of Salt.
Gradually add cold Water while whisking until you have a smooth, slightly thin pancake batter; then beat in the Egg until incorporated. Let the batter rest 10–15 minutes to hydrate the flours.
Meanwhile, prepare the mix-ins: cut the Scallions into long 2–3 inch pieces and slice the Red Chili thinly. Pat the Mixed Seafood dry so it won’t water down the batter.
Fold the scallions, seafood and a few slices of red chili into the rested batter so they are evenly distributed but not overloaded.
Heat a large nonstick or well-seasoned skillet over medium-high heat and add a thin layer of Neutral Oil. When the oil shimmers, pour or ladle in enough batter to form a thin pancake (adjust size to your pan).
Use the back of the ladle or a spatula to spread the batter into an even circle. Cook undisturbed until the edges look dry and the underside is deep golden and crisp, 3–5 minutes.
Flip carefully (or flip in two stages if the pancake is large) and cook the second side until golden and the seafood is cooked through, another 2–4 minutes. Press gently with the spatula for uniform contact with the pan so the pancake crisps evenly.
Transfer the finished pancake to a paper towel–lined plate to drain briefly, then slice into wedges. Repeat with remaining batter, adding more oil to the pan as needed.
Make the dipping sauce: combine Soy Sauce, Rice Vinegar, a splash of Water, a pinch of Sugar and a pinch of Gochugaru to taste. Stir in a few drops of Toasted Sesame Oil and finish with toasted sesame seeds for texture.
Serve the haemul pajeon hot with the dipping sauce on the side; garnish with extra sliced scallion or red chili if you like. Enjoy immediately for the best crispness.
