While cabbage and radishes marinate, slice onion, green onion, red pepper, garlic, and ginger into ¼-inch wide slices
Drain salted liquid from cabbage and radishes, saving the salted liquid.
Fill a large glass jar or container halfway with cold, clean water. Add the drained salty liquid from the vegetables. Stir in 2 tablespoons sea salt, 2 tablespoons Korean red chili pepper powder, 1 tablespoon sugar, and 1 tablespoon fish sauce if using.
Add all of the sliced cabbages, radishes, onion, green onion, and bell pepper. Give the jar a good stir to make sure all of the salt is dissolved.
Cover and place in a cool spot on the countertop for 1 day, then store in the refrigerator.