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1
Inspect each perilla leaf and discard any that show yellowing or soft spots, because a leaf already softening on the stem will turn to mush long before the brine has had time to do its work. Wash the leaves and pat them fully dry so that surface water does not dilute the brine from the first pour.
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2
Combine the ganjang and water in a small saucepan in a 1:1 ratio and bring to a simmer, stirring once to marry the two liquids, then remove from the heat and allow the brine to cool completely to room temperature. The brine must be cold before it touches the leaves — hot liquid will wilt and cook them rather than slowly draw flavour inward, leaving you with something closer to braised greens than a firm, seasoned pickle.
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3
Layer the perilla leaves in a clean jar or lidded container in sets of 2–3 leaves, keeping the layers loose so the brine can find its way between every sheet rather than sheeting off in one direction. Pour the cooled brine over the leaves until all are submerged, then leave the container unsealed at room temperature for 24 hours so the brine can begin its work — refrigerating at this stage arrests penetration before it has properly started.
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4
After 24 hours, drain the brine into a saucepan, bring it back to a full boil, and allow it to cool completely before re-pouring it over the leaves — this is where the dish lives or dies, because repeating this drain, re-boil, cool, and re-pour cycle 3–4 times over the course of a week progressively concentrates the brine, drives it evenly through every layer, and keeps the leaves that deep, jewelled green rather than dull and grey. A single pour leaves the outer leaves brash with salt while the inner ones taste of almost nothing.
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5
On the final pour, stir the gochugaru, minced garlic, sesame seeds, and sesame oil into the re-boiled and fully cooled brine before pouring it over the leaves, so the aromatics are carried into the layers by the liquid rather than sitting only on the surface. After this final pour, seal the container and transfer it to the refrigerator.
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6
The jangajji is ready when the leaves are uniformly darkened, pliable but not mushy, and carry a deep soy-sweet-sesame complexity throughout — not raw at the centre, not saline to the point of harshness. Taste a leaf pulled from the middle of the jar to confirm even penetration before serving.