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1
Soak the glutinous rice in cold water for 1 hour so the grains absorb enough moisture to steam evenly without turning chalky at the centre. Drain, then steam over high heat for 15 minutes to par-cook — the grains should be tender at the surface but retain a firm core, as they will finish cooking inside the squid.
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2
Soak the glass noodles in warm water until fully pliable, then drain and cut into short lengths. Blanch the mung bean sprouts briefly, drain, and squeeze them dry in a cloth so they release no water into the filling — excess liquid would make the rice gummy and weaken the seal during steaming. Combine the par-cooked rice, noodles, and sprouts in a bowl, add the ganjang, 1 tablespoon of the sesame oil, and the shredded perilla leaves, and mix until the seasoning is evenly distributed, since the sealed squid casing will trap every flavour inside and there is no way to adjust once it is closed.
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3
This is where the dish lives or dies. Pack the filling into each cleaned squid tube to no more than 80% of its capacity, because the filling expands during steaming and an overfilled tube will split at the seal, losing everything into the steaming water. Seal the open end by threading closely spaced toothpicks across the opening, or by sewing shut with food-safe thread, making sure the closure is firm and without gaps.
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4
Arrange the sealed squid tubes in a steamer basket in a single layer so the steam circulates around each tube and cooks them at the same rate. Steam over high heat for 20–25 minutes — the squid is done when the exterior has turned fully opaque and orange-pink and the tube feels firm but yields slightly under finger pressure. Do not reduce the heat partway through, as a faltering steam will toughen the squid wall before the filling has time to heat through.
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5
Remove the squid from the steamer and, while still hot, brush the exterior lightly with the remaining sesame oil so the surface does not dry and tighten as it cools, and to give a finishing sheen. Allow to rest for 2 minutes before slicing, so the filling settles and holds its shape cleanly when cut.
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6
Remove the toothpicks or thread, then slice the squid into rounds approximately 1.5 cm thick. Arrange cut-face-up on a plate so the cross-section — the orange-pink squid ring encircling the filling — is fully visible. Serve immediately with cho-ganjang and the doenjang-salt mixture alongside.