Grate the carrots on a coarse grater or chop them by passing them through a food processor on the Korean carrot setting. Cut the onion into half rings, then chop them randomly with a knife.
Place the grated carrots in a deep bowl, add sugar and salt.
Add the onion to the carrots, mix the salad and wait until all the salt and sugar crystals have dissolved.
Transfer the salad to a jar, packing the vegetables tightly. Cover with a cotton napkin - a piece of cloth or a paper towel folded in half, secure with a rubber band and put in the refrigerator for a week.
The salad is ready to eat within a few hours of marinating, but after a week its flavor becomes richer. Before putting the jar away for long-term storage, close it tightly with a metal lid.