In a medium saucepan, combine sugar, apple cider vinegar, garlic powder, turmeric, and mustard seed. Bring to a boil over medium-high heat, stirring until the sugar dissolves.

Add the sliced chiles to the boiling syrup. Reduce heat to medium and simmer for 5-7 minutes, until they soften slightly, and the jalapeños turn a darker green.

Remove from heat and let cool slightly. Using a slotted spoon, transfer to a clean jar.

Pour the hot syrup over the chiles in the jar, leaving about ¼ inch of headspace. Seal the jar with a lid.

Let the jar cool to room temperature, then refrigerate for at least 24 hours before eating. The flavors will develop further over time!