Wash the cabbage thoroughly in plenty of cold water, then slice in half lengthways. Cut each half lengthways again into long quarters and then a final time into eighths, making sure to keep the stem intact.
Sprinkle each piece with the sea salt, making sure to work the salt in between all the leaves, then place all the cabbage pieces in a large bowl. Pour over just enough cold water to cover the cabbage, arrange a plate on top and weigh it down with a couple of tins. Leave for 2 hours to brine.
Meanwhile, make the marinade by mixing together the garlic, ginger, gochugaru, fish sauce, radish and spring onions until well combined. Set aside.
Once the cabbage has sat in the brine for 2 hours, drain it well in a colander, then rinse for several minutes under cold running water. Shake off the excess water in the colander, then gently squeeze the leaves to remove as much water as possible. Place the drained leaves in a large bowl, pour over the marinade, then using your hands (plastic gloves recommended), work the paste well into each layer of leaf.
Pack the marinated leaves into a 1-litre (34fl oz/4 cups) Mason jar or non-reactive (e.g. glass) container, press them down well with the back of a metal spoon and cover.
Leave for 5 days at room temperature. The cabbage will become submerged in a red liquid – use the back of a spoon to press the leaves down under the liquid each day before replacing the lid. After 5 days, refrigerate the kimchi and allow it to mature for a further 2–3 weeks. It will keep well for a couple of months refrigerated and its strength will increase the longer it is left.
Alphabet Cooking: K is for Korean (Quadrille, £12) Photography Kim Lightbody
Visit our Asian recipes section for recipes from Korea, China, Japan and more, including Korean tofu and kimchi stew, kimchi jjaegi.