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1
Combine the rice vinegar, soy sauce, mirin, and dashi in a small saucepan. Bring to a bare simmer, stirring once to integrate. Add the sliced chilli and julienned vegetables. The liquid should smell sharply acidic with a round, sweet undertow — remove from the heat and hold warm.
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2
Heat the oil in a deep pan to frying temperature. Working in small batches, lower the fish into the oil. Fry until the skin is audibly crackling and the fish are rigid and deeply golden — this is where the dish lives or dies: the surface must be genuinely crisp and porous so the marinade can penetrate as the fish cool; an underdone fry produces a sodden, insipid result.
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3
Lift the fish immediately from the oil and transfer them in a single layer to a non-reactive dish. Pour the warm marinade and vegetables over the hot fish at once. The sizzle will subside quickly; the fish should be fully submerged or closely covered by the liquid.
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4
Allow the fish to cool to room temperature, turning once. The flesh will have absorbed the marinade visibly — the surface colour shifting from golden to a deeper amber-soy tone. Cover and refrigerate. The dish is ready to eat once fully cold but improves over the following days; taste on day two for the fully balanced sweet-sour-salty result.