01 -
Put the sliced jalapeños and onions into a clean, heat-safe glass jar. Pack them in nice and snug.
02 -
In a small saucepan, mix together both vinegars, sugar, celery seed, mustard seeds, and salt. Give it a good stir to combine everything.
03 -
Bring the mixture to a rolling boil over medium-high heat, stirring until the sugar and salt completely dissolve - this takes about 3-4 minutes.
04 -
Carefully pour that hot brine right over the jalapeños and onions, making sure everything is completely covered with liquid.
05 -
Let the jar sit uncovered on your counter for about an hour to cool down to room temperature.
06 -
Pop the lid on and stick it in the fridge for at least 4 hours, but overnight is even better for the best flavor.