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1
Bring a large pot of water to a vigorous boil and add the salt, so the water is seasoned and the spinach enters at full temperature without dropping the boil. Prepare a bowl of ice water large enough to submerge the spinach entirely.
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2
Add the spinach to the boiling water and blanch for exactly 30–45 seconds, watching for the moment the leaves are uniformly wilted and intensely bright green — that colour is the signal that the heat has done its work without beginning to dull. Not a second longer; 60 seconds is sufficient to destroy both colour and texture.
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3
Transfer the spinach immediately to the ice water so the residual heat inside the leaves stops carrying the cook forward on its own. Hold for 30 seconds, no more, to arrest the cooking precisely.
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4
Lift the spinach from the ice water and squeeze it firmly between both hands; then squeeze again, so no water remains to dilute the dressing and turn the finished dish flat and wet. This is where the dish lives or dies — properly squeezed spinach forms a firm, compact ball with no water dripping from it; insufficient squeezing produces a diluted, flat dressing and a disagreeable wet texture.
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5
Set the squeezed spinach aside at room temperature for a few minutes, so the chill from the ice bath eases off and the sesame oil has a surface it can cling to rather than sliding away. It should be slightly cool but not refrigerator-cold before dressing.
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6
Combine the guk-ganjang, sesame oil, minced garlic, and sugar in a bowl, then add the spinach and toss, separating the strands with your fingers so the dressing reaches every leaf evenly rather than pooling in the centre of a compressed clump. The finished namul should look uniformly glossy and smell of sesame and garlic.
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7
Transfer to a serving dish and scatter the toasted sesame seeds over the surface. Taste: the namul should be clean and bright, with no raw edge and no wateriness — adjust with a few drops more guk-ganjang if required.