Place clean bok choy in a large mixing bowl. Dissolve the sea salt in water and pour over to bok choy. Let it sit for 30 minutes, then drain.
To make the kimchi seasoning, puree onion, apple, and garlic with 1/4 cup of sea kelp stock in a blender until smooth. Add the sliced red chilies and pulse the mixture until the chilies are processed into small pieces.
Put the pureed mixture into a medium mixing bowl. Add the Korean chili flakes, anchovy sauce, and ginger puree. Mix well.
Toss the bok choy with the kimchi seasoning paste. Be gentle when you are coating the bok choy with the paste so that you don't bruise them.
Bok choy kimchi tastes best within 2-3 days, as it is not meant for long-term storage like most Korean kimchi. If there's any leftover, store it in an airtight container in the refrigerator. If you prefer it fermented, leave it at room temperature for one day before refrigerating. I recommend consuming this kimchi within 2 weeks if fermented.
Notes
To make the sea kelp stock; simmer a piece of dried sea kelp in a cup of water for 4-5 minutes. Discard the kelp and let the stock cool down.Want a fish sauce-free option? Try Korean soup soy sauce for a delicious vegetarian alternative with rich flavor!