In a medium saucepan, combine apple cider vinegar, brown sugar, salt, and peppercorns. Bring to a boil over high heat, stirring occasionally until the sugar and salt dissolve.
Add the sliced red onion and sliced jalapeño to a clean, airtight glass container.
Pour the hot pickling mixture over the onions until they are completely submerged.
Tightly seal the container and refrigerate for at least 30 minutes or overnight before serving.
Notes
Pickled red onions can be stored in the refrigerator for up to 3 weeks.