Chop the onion into a small dice. Remove the seeds and chop the jalapenos into a small dice.
Add vinegar, sugar, and salt to a small saucepan over medium-high heat.
Once the liquid is simmering, add the finely diced vegetables and cook for 10 minutes.
After 10 minutes, the vegetables should have softened so you can turn off the heat and pour the vegetables and liquid into a sealed container. Once everything has cooled, store it in the fridge for up to a month.