Rinse the radishes and carrots and cut off any small hairs on the skin. (You can also peel the skin, I leave it on.)
Cut the radishes and carrots into bite sized cubes and put them in a large bowl.
Coat the mixture with sea salt evenly.
Let it rest for 30 minutes so they have time to sweat.
Drain out most of the juice but leave about 2 tbsp to 1/4 cup in the mixture.
Add minced garlic, grated ginger, fish sauce, red pepper flakes, and chopped onions.
Mix everything well so the seasonings are evenly coated.
Transfer to a glass jar with an airtight lid, pressing down the mixture gently as you pour it in.
Close the lid and let it sit outside the fridge at room temperature out of sunlight for 24-48 hours while it ferments.
It's ready when you open the lid and you see small bubbles escaping off the top and the kkadugi has a strong, sour smell.
Store in the fridge and eat it with EVERYTHING.