Prepare the onions by using a mandolin or a sharp knife to slice them into very thin circles.
Stuff the onions into the jar. You can pack them in pretty tightly.
In another jar, combine the vinegar, hot water (I just use tap water set to the hottest level), sugar, and salt. Seal tightly. Shake until the sugar and salt are dissolved. Alternatively, you can put the brine ingredients in a mixing bowl or measuring cup and use a whisk to stir it until everything is dissolved.
Pour the brine over the onions. It may not fully cover them, but this is OK, because the onions will start to sink down while sitting in the brine. Seal the jar with the lid. Let the onions sit for about 30 minutes so that they come to room temperature.
Enjoy right away or store in the fridge for later use. They will get more flavorful the longer they sit in the brine.
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Notes
These are not shelf-stable/canned pickles, so they need to be stored in the refrigerator. Use within 2 weeks.
Pickled red onions taste great on sandwiches, tacos, gyros, burgers, charcuterie boards, nachos, and more.