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1
Combine the plain flour and starch in a bowl, then add the ice-cold water and mix with minimal strokes — the cold temperature slows gluten formation, so the finished pancake stays shatteringly crisp rather than chewy and dense. Stop the moment no dry flour is visible; some lumps remaining is correct, and overmixing now will produce a tough, bread-like result. The batter is ready when it coats a spoon in a thin, loose sheet.
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2
Lay the green onions across the surface of the batter rather than stirring them in, so they lie in a neat, parallel layer that will face the pan and char cleanly. Arrange the squid, shrimp, and oysters on top of the onions in a single layer. Keep all seafood at the surface; anything submerged will steam in the batter instead of searing against the hot oil.
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3
Heat a wide, heavy pan over high heat until very hot, then add enough oil to shallow-fry — the surface should shimmer and the oil must move freely when the pan is tilted, so the batter lifts and crisps rather than sticking and absorbing fat. Pour the loaded batter into the pan in one motion, spreading it to an even round, then press flat immediately with a spatula and hold firm. The oil should hiss loudly on contact.
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4
Cook until the batter has set 60–70% through — the edges are opaque and pulling away from the pan, but the centre remains slightly translucent. Stream the whisked egg evenly over the entire surface at this point, so the batter is firm enough to hold the egg on top rather than letting it sink and steam into the interior, where it would turn the pancake heavy and wet. The egg is ready to proceed when it has turned from glossy-wet to just-set and no longer flows when the pan is tilted.
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5
Flip the pancake once, in a single confident motion, then immediately press firmly with a spatula and hold for 15 seconds so the egg side makes full contact with the oiled pan and colours evenly rather than blistering in patches. Cook until the egg side is golden and the underside is deeply coloured and rigid at the edges. The pancake is done when it slides freely and the underside, lifted at one edge with a spatula, shows an even, amber-brown crust.
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6
Slide off the heat immediately and transfer to a board or plate. Stir the soy sauce and rice vinegar together to form the cho-ganjang. Serve the pancake whole or cut into portions, with the dipping sauce alongside.