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1
Combine the rice vinegar, soy sauce, mirin, and sugar in a shallow non-reactive dish wide enough to hold the fish in a single layer. Add the togarashi. Stir until the sugar dissolves. The marinade is ready when the liquid is clear and uniformly combined.
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2
Blanch the sliced onion, carrot, and pepper briefly in boiling water — no more than 30 seconds. Drain and add immediately to the waiting marinade so the vegetables begin absorbing the liquid. They should be just softened at the edges but retain body.
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3
Dry the fish or shrimp thoroughly. Dust evenly with plain flour, shaking off any excess. Heat the oil in a suitable vessel to frying temperature. Fry the fish in uncrowded batches until the exterior is deeply golden and audibly crisp — a clean, sustained sizzle that does not steam. The coating must be rigid to the touch when lifted with tongs.
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4
This is where the dish lives or dies: the moment the fried fish leaves the oil, transfer it — while still at full frying heat — directly into the marinade. The hot fish steams slightly against the cold vinegar liquid; this heat exchange drives the sweet-sour-savory marinade deep into the flesh. Fish that has cooled before entering the marinade will not achieve the same depth of penetration.
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5
Leave the fish submerged in the marinade at room temperature for a minimum of 30 minutes; 2 to 4 hours is optimal. The fish is ready when it is tender and fully seasoned to the centre, yet retains a degree of crunch at the thicker sections of crust. Taste a piece: the balance of sweet, sour, and savory should be present in a single bite. Do not exceed 12 hours of marination, at which point the acid begins to break down the texture to an unacceptable mealiness.