Slice jalapeños into thin rounds. If you prefer less heat, remove some or all of the seeds. We leave them in. Set sliced jalapeños aside.
In a medium saucepan, combine sugar, apple cider vinegar, mustard seed, celery seed, turmeric, and garlic powder. Bring to a boil over medium-high heat, stirring until the sugar dissolves. Reduce heat and simmer for 5 minutes.
Add the sliced jalapeños to the syrup and simmer for 4–5 minutes, just until they begin to soften and look glossy.
Using a slotted spoon, transfer the jalapeños into jars. Return the syrup to a boil and then simmer for an additional 2-3 minutes. Remove syrup from heat and carefully pour the hot syrup over the top to fully submerge the peppers. Let cool to room temperature, then seal with lids and refrigerate.
Chill for at least 24 hours before using (if you can wait that long!). They only get better as they sit.
Storage: Store in the refrigerator in a sealed jar for up to 2–3 months. The flavors deepen over time, so they're even better after a few days!