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1
Combine the sweet potato starch and cold water in a bowl, whisking until fully dissolved and no lumps remain. Add the drained oysters and sliced spring onions to the batter and fold gently to coat. The batter will be thin and will settle quickly — stir again just before use. It is ready when the starch is fully suspended.
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2
Set a wide, heavy pan or flat-bottomed wok over high heat. Add the lard and allow it to melt and shimmer fully before proceeding — do not crowd the pan. Pour the oyster batter in a single, even layer and press it lightly to settle the oysters. Cook undisturbed on high heat until the underside is set and deeply caramelised: the edges will be visibly crisp and pulling from the pan, and the surface will have shifted from opaque white to translucent. This is where the dish lives or dies — insufficient heat or a crowded pan will steam rather than fry, and the characteristic crust will not form.
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3
Break the eggs directly onto the surface of the cake, one by one. Using a spatula, break the yolks and spread the egg loosely across the top, then fold the cake over itself, incorporating the egg into the interior. The egg should be just set — custardy rather than dry — and the interior should remain noticeably sticky and chewy when pressed with the spatula.
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4
Flip the folded cake and press it briefly against the pan surface to re-crisp the exterior for thirty seconds. The finished cake should resist the spatula slightly — springy and cohesive, not dissolving — and carry a caramelised, lightly smoky aroma. Plate immediately and serve with sweet chilli vinegar sauce alongside.