Step 1 Remove the outer leaves of the napa cabbage and cut the cabbage into quarters lengthwise. Rinse the cabbage thoroughly between the leaves and dry the cabbage well.
Step 2 Salt the cabbage, being sure to spread the salt evenly between the leaves. Place the salted cabbage in a plastic or non-reactive colander or strainer and place the colander over a bowl to collect any liquid. Set the cabbage aside for 30 minutes.
Step 3Meanwhile, combine the gochujang paste, chile flakes, water, vinegar, chopped garlic and grated ginger in a food processor, and pulse the ingredients to form a coarse paste.
Step 4Rinse the salted cabbage under cold running water, making sure to rinse the salt out from between each leaf. Dry the cabbage completely on clean towels. Cut each quarter crosswise into 2-inch to 3-inch strips, discarding the core. Place the cabbage in a glass or other non-reactive bowl and toss with the onion, radish and green onion.
Step 5Add the paste to the bowl, massaging it into the cabbage leaves and vegetables. Place the mixture into a deep non-reactive container (such as an 8-cup measuring cup), and weight the mixture down. Refrigerate 1 to 2 days before using to give the flavors time to ripen. This makes a scant quart of kimchi, which will keep for up to a week, refrigerated.