01 - Tightly pack the sliced red onions into a clean, heatproof 2-cup jar. Distribute the sliced garlic, red pepper flakes, and black peppercorns throughout the jar.
02 - In a small saucepan over medium heat, combine apple cider vinegar, water, hot honey, granulated sugar, and kosher salt. Stir constantly until the mixture reaches a gentle simmer and the sugar and salt have completely dissolved.
03 - Carefully pour the hot brine over the onions, ensuring they are completely submerged. Use the back of a spoon to press down any floating onion slices.
04 - Allow the jar to cool to room temperature uncovered. Cover tightly with a lid and refrigerate for at least 1 hour before serving. For optimal flavor development, marinate overnight.
05 - Store refrigerated for up to 2 weeks. Ensure onions remain submerged in brine for maximum freshness.