Roughly peel the radish, remove the top and bottom roots, and clean out the holes and dirt on the surface.
Slice it into 1-inch discs, then cube it in 1x1x1-inch bitesize pieces.
In a bowl or container, layer radish and sea salt alternately until all of the pieces are salted.
Dissolve the new sugar in room temperature water then add in the radish. Make sure it is submerged well, then set it aside to brine.
Meanwhile, chop the scallions and mustard greens. Place it in a bowl and add the gochugaru, then set aside.
Blend the garlic, onion, ginger, cooked rice, salted shrimp, and fish sauce.
Add the blended sauce to the vegetables with gochugaru, and set aside.
Drain the radish and run it in ONLY a cup of water to wash down the excess salt. Do not wash it in running water, just drain it thoroughly.
Add the radish to the sauce and vegetable mixture, then mix thoroughly. Make sure every piece is coated in the sauce.
Transfer to a kimchi container or glass jar and leave to ferment at room temperature for 5 days or until desired sourness is reached. Taste as you go!
When desired sourness is reached, transfer to the fridge to slow down the process. Peak taste will be in a week or two, and it can still be stored for several months.
Serve as a side dish to soups or turn into fried rice or jjiigae.