Bring a pot of water to a boil. Add the onions and cook for 30 seconds (this quick cooking is called blanching). Drain the onions in a colander.
Return the onions to the pot and add the vinegar, sugar, and salt. Add enough water to just cover the onions. Bring to a boil over high heat, then transfer the onions and the liquid to a glass container and cool. You can use the onions right away or seal the container and refrigerate them for up to 4 months.
Notes
Store in the fridge in a sealed glass container. Metal can react with the vinegar, and plastic might absorb the flavors, so if you go with plastic, know that the container might always be your pickled onion container. You can use the onions right away or seal the container and refrigerate them for up to 4 months. They probably have the optimal flavor and texture between weeks one and four.