On the stovetop use a small to medium sized sauce pan. Add the red wine vinegar and brown sugar. Heat on medium low. Just heat up until warm, not hot or boiling. You want the sugar to dissolve. This should only take a few minutes and should be safe for you to test with your finger for the temperature.
Using a mandolin or sharp knife, thinly slice the onion I use a setting of 2 or 3.
Place onions into an air-tight container like a mason jar. Do not seal with lid.
Pour warm vinegar and sugar mixture on top. Seal with the lid.
Place the jar on a counter at room temperature for about 1-2 hours. Although they should be good to go at 30 minutes, the longer they sit out the more pickled they become.
Move to the refrigerator and keep stored with lid sealed tight for up to 3 weeks, best enjoyed within a week.
Notes
- These can last for up to 3 weeks in the refrigerator if kept in a tightly sealed jar. However, they usually taste the best within about 3-6 days after making.