Using a mandolin, slice onion on the thinnest setting. Slice jalapenos, thicker, about the width of half a pencil.
Stuff onions, jalapenos, and the clove of garlic into a small jar; you want it to be as packed as possible.
Combine vinegar and sugar, stir to dissolve sugar, and pour into container.
Leave in the refrigerator for 5 days. Shake a flip jar once a day. It will remain fresh for a month.