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1
Ensure the dried sardines carry no residual moisture — spread them on a tray and leave uncovered if any softness is apparent. Place a dry frying pan over medium heat. Add the sardines in a single layer and toast, moving them constantly, until they are uniformly rigid and snap cleanly when bent. Remove immediately from the pan and set aside on a flat surface.
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2
In the same pan, combine the soy sauce, mirin, sake, and sugar over medium heat. Bring to a gentle boil, stirring briefly to dissolve the sugar. Reduce the heat and continue cooking without stirring until the syrup thickens visibly. Test by dipping a chopstick into the syrup and lifting it clear: if a thread of 2–3 cm forms before breaking, the glaze has reached approximately 115 °C and is ready. This is where the dish lives or dies — a glaze reduced too little will not lacquer the fish; it will merely moisten and soften them.
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3
Return the toasted sardines to the pan all at once. Toss swiftly to coat every fish with the glaze, then transfer the entire contents immediately onto a sheet of parchment paper. Working quickly with chopsticks or a fork, separate each sardine into a single layer before the syrup sets. The window for separation is short — prolonged handling causes clumping that cannot be undone.
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4
Scatter sesame seeds over the arranged sardines while the glaze is still tacky. Leave undisturbed at room temperature until fully cooled. The glaze will harden to a lacquer-like finish; each sardine should be individually crisp, glossy, and separate from its neighbours. Taste one: the texture must be brittle, not yielding.