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1
Combine soy sauce, mirin, sake and sugar in a small saucepan. Add the reserved eel bones and heads. Bring to a simmer and reduce by 30 percent, then strain. This is the tare. A newly made tare is serviceable; a tare built over successive batches — each fresh reduction added to the strained mother sauce — develops the complexity that defines great kabayaki. The sauce should coat a spoon and carry a deep, caramelised soy fragrance.
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2
Thread bamboo or metal skewers lengthwise through the butterflied eel fillets to keep them flat and prevent curling during grilling. The flesh should lie perfectly level.
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3
Grill the skewered eel over live charcoal, skin side down first, to medium rare. The surface should show faint colour and the flesh should be just set but not cooked through. Remove from the heat.
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4
Transfer the part-grilled eel to a steamer and steam for 8 to 10 minutes. This is where the Tokyo-style dish lives or dies: the steam releases excess fat from the flesh, and the heat penetrates gently to produce the characteristic pillow-soft, yielding interior that defines Kanto kabayaki. Eel that has not been steamed will be dense and the fat will not have cleared; properly steamed eel yields to the lightest pressure and the skin slips from the flesh without resistance.
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5
Return the steamed eel to high charcoal heat. Apply the tare by brushing generously. Grill for 2 to 3 minutes, applying a second coat of tare mid-way. The glaze should caramelise to a lacquered, deep-amber surface with a faint char at the edges and the scent of sweet soy and smoke rising sharply from the grill.
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6
Divide the cooked rice among four lacquer boxes (unaju). Lay the glazed eel portions over the rice. Close the lid for a moment before serving — the trapped steam keeps the eel soft. Sprinkle sansho pepper over the eel only at the point of service; applied earlier, its delicate numbing citrus-pine compounds are lost to heat.