Separate the chopped onions and place them in a sterilized glass jar. (Add some sliced chilies between layers if you opted to include them.)
Combine the water, soy sauce, vinegar and sugar in a saucepan and boil over low to medium heat until the sugar dissolves (about 2-3 minutes). Stir often.
Pour the brine over the onion and close the lid. Leave at room temperature for 3 to 4 hours then refrigerate. It should be ready to eat in 1 to 2 days.
Serve with your favorite main meal (e.g. Bindaetteok or Korean BBQ). It can be refrigerated in an airtight container for at least one month.
