Bring 2 to 3 cups water to a boil in a small saucepan. Combine the vinegar, salt and sugar in a sealable container and stir to dissolve.
Peel and thinly slice the onions. Place the onions in a strainer in the sink. Slowly pour the boiling water over the onions and let them drain. Add the onions to the container and stir, or shake if sealed, to evenly distribute the flavorings. Refrigerate. The onions will be ready in about 30 minutes, but are better after a few hours. They will keep for several weeks but are best in the first week.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.