How to Make It
Place jalapeño chiles in a medium bowl or jar. Whisk together apple cider vinegar and kosher salt in a small saucepan over medium heat. Bring to a boil. Remove from heat; immediately pour vinegar mixture over chiles. Let cool completely. Store, covered, in refrigerator at least 8 hours or overnight. (For best flavor, make several days in advance.)
When ready to serve with oysters, thinly slice a few of the chiles, and place slices in a serving bowl. Add some of the vinegar, and stir in fresh cilantro before serving with oysters.