This doesn't sound Korean. It doesn't taste Korean. But it goes unbelievably well with this Korean meatloaf. Makes no sense but it totally works.
Soak the onion in cold water for 5 minutes. Drain well.
Heat the rice wine vinegar and sugar over low heat, stirring regularly until the sugar has dissolved completely. No need to let it come to a boil. As soon as the sugar melts, you're done.
Let the rice wine vinegar mixture cool. Then stir in the mustard.
Toss the mixture with the onions. Let it stand 6-8 hours at room temperature or overnight in the refrigerator. Do not taste it immediately. Just don't do it. The mustard is hot. It mellows as it stands but at first it will blow a hole in the back of your head. Think about the last time you had too much wasabi. Same effect exactly.