Wash and slice your jalapeños and onion. Wear gloves if you’re sensitive to the heat.

In a medium saucepan, combine the sugar, vinegar, water, mustard seeds, celery seeds, turmeric, and salt. Bring to a gentle boil over medium heat, stirring until the sugar dissolves.

Once the brine is ready, add the sliced jalapeños and onions. Stir for 1-2 minutes to coat the veggies.

Pour the jalapeño and onion mixture into clean, sterilized jars, ensuring the vegetables are covered with brine. Leave about a half-inch of headspace.

Seal the jars tightly and let cool at room temperature for 30 minutes, then refrigerate for at least 24 hours before tasting.