01 - Pack the sliced red onions into a clean, heatproof 2-cup jar. Add the garlic, red pepper flakes, and peppercorns.
02 - In a small saucepan, combine apple cider vinegar, water, hot honey, sugar, and salt. Bring to a simmer over medium heat, stirring until sugar and salt dissolve.
03 - Pour the hot brine over the onions in the jar, ensuring they're fully submerged. Press down with a spoon if needed.
04 - Let cool to room temperature, then cover and refrigerate for at least 1 hour before serving. For best flavor, marinate overnight.
05 - Store refrigerated for up to 2 weeks.