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1
Fillet each fresh-killed eel using the anago-oroshi technique: make a clean entry cut behind the head, then slide the knife along the spine to open the fillet. Remove the backbone and trim. The fillet should lie flat and even, with the skin intact.
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2
Blanch the fillets briefly in hot water — the shimo-furi step. Submerge each fillet for a short time until the skin firms and any surface sliminess is released. Lift immediately. The skin should feel firm and clean to the touch, and the surface should show no remaining slick residue.
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3
Combine the dashi, soy sauce, sake, mirin, and sugar in a wide, shallow pan. Bring to a gentle simmer. Add the blanched anago fillets skin-side up. Cover with an otoshibuta (drop lid) and simmer over low heat for 10–12 minutes. This is where the dish lives or dies: at 10 minutes, test by passing a chopstick through the thickest part of the fillet — it must pass through with almost no resistance, yet the fillet must still hold its shape. A spring-back indicates undercooking; complete absence of any structure indicates overcooking. Either fault cannot be corrected at service.
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4
Remove the fillets carefully and set aside to cool. Strain the cooking liquid through a fine sieve into a separate pan. Reduce over moderate heat, stirring to prevent burning, until the liquid thickens to a dark, glossy syrup that coats the back of a spoon — approximately 5–10 minutes of active reduction. The tsume is ready when it holds a clean line drawn through it on the spoon.
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5
Portion the cooled anago fillets to match the shari base. Before service, bring the portioned anago to slightly above room temperature — warm through, so the fat is just beginning to melt. Cold anago, with its fat solidified, produces an unpleasant waxy texture entirely contrary to the dish's intent. The surface of the eel should appear faintly glistening.
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6
Form the shari into warm nigiri bases. Lay one anago portion skin-side up on each. Brush generously with hot tsume, ensuring an even, adherent coat across the full surface. The tsume must be thick enough to remain on the eel without pooling onto the rice — a thin, watery glaze indicates insufficient reduction.
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7
Serve immediately. The anago should dissolve on the palate into a smooth, silky layer with no resistance, carrying the concentrated sweet-savoury complexity of the tsume. Any firmness or waxiness signals a failure of temperature or cooking.