01 - Place the sliced red onions into a clean, heatproof 2-cup jar. Add the sliced garlic, red pepper flakes, and whole black peppercorns on top.
02 - Combine apple cider vinegar, water, hot honey, granulated sugar, and kosher salt in a small saucepan. Heat over medium heat, stirring constantly until the sugar and salt completely dissolve. Bring to a gentle simmer.
03 - Carefully pour the hot brine over the onions in the jar, ensuring all onion slices are fully submerged. Press down gently with a spoon if needed to release air pockets.
04 - Allow the jar to cool to room temperature, then cover tightly with a lid. Refrigerate for at least 1 hour before serving. For optimal flavor development, marinate overnight.
05 - Keep refrigerated and use within 2 weeks for best quality and flavor.