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1
Source live or just-killed blue swimming crab and proceed within 2 hours of death, so the flesh has not had time to slacken and sour from the inside out. Rinse each crab under cold running water, remove the carapace, discard the gills, and clean the body cavity, taking care to preserve the roe (내장, naejang) intact inside the shell, then halve the body through the centre. The cleaned crab should smell clean and of the sea — any ammonia or sourness signals unsalvageable deterioration.
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2
Combine the ganjang, water, garlic, ginger, chilli, and pear in a saucepan and bring to a full boil, then remove from the heat — boiling drives off the raw, acrid edge of the aromatics and lets the pear's sweetness marry into the brine rather than sit on top of it. Allow the marinade to cool completely to room temperature before it touches the crab, so the heat does not begin to cook the flesh and rob it of its silky, raw character. It should carry a clean, sweet-savoury aroma with no raw edge remaining.
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3
Arrange the cleaned crab pieces in a clean, non-reactive container roe-side up, so the naejang sits cupped in the shell and stays submerged rather than floating loose and exposed. Pour the fully cooled marinade over the crab, ensuring all pieces are submerged, then cover and refrigerate for 24 hours. The marinade will begin to take on a paler, slightly diluted colour as moisture is drawn from the crab.
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4
On day two, pour off the marinade into the saucepan, bring it back to a full boil, then cool completely — because re-boiling re-concentrates the brine that the crab has diluted, so each subsequent cycle drives flavour deeper into the flesh rather than merely sitting on its surface. Pour the re-concentrated marinade over the crab again and refrigerate for a further 24 hours. Repeat once more on day three — two to three repetitions in total — and each cycle the marinade will deepen in colour and the crab flesh will firm, then relax into its characteristic silky texture.
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5
After the final cycle, bring the gejang to room temperature before serving, so the roe softens from its fridge-cold set into the creamy, yielding paste it should be at the table. The crab meat should slide freely from the shell with a silky, near-liquid quality, and the roe inside the carapace will have set into an intensely savoury paste. Serve the crab body and carapace over or alongside hot steamed rice; hold the carapace over the rice bowl and scoop the roe paste directly onto the rice with chopsticks, then pour the reserved marinade over the rice as desired.