Cut the Chinese leaves into quarters lengthwise and soak in lightly salted water for 1 hour.
Put the Gochujang, chopped onion, garlic, peeled ginger, salt and sugar, oyster sauce, soy sauce, rice wine vinegar and water into the food processor/blender and blitz until lovely and smooth.
Drain the cabbage and transfer to a rimmed tray/baking sheet, then use a spoon or a brush to thoroughly coat the cabbage layer by layer with the paste.
After each layer is covered, massage the paste all over the cabbage, covering as much as possible with the paste. Repeat with the rest of the cabbage quarters.
Once all the leaves are covered, add the chopped spring onions and place them within each layer of the cabbage.
Put some of the paste on the baking sheet and roll the cabbage quarters like a log in the paste and covering completely.
Once the cabbage has been rolled and coated in the paste, cut the cabbage into chunky portions (depending on the size of the jar) and to the exposed areas with the paste.
Carefully layer the pieces of cabbage into the jar making sure the layers of the cabbage stay together. Jam in any remaining pieces of cabbage to the top and then top it with leftover paste. Gentle push everything down to make sure everything is covered and settled.
Let it sit at room temperature for 2 days. Be aware that natural gasses build up so you may need to burp the jars each day to release them! After two days put the jars in the fridge for 2 weeks.