01 - Pack the thinly sliced red onions into a clean, heatproof 2-cup jar. Add the sliced garlic clove, red pepper flakes if using, and whole black peppercorns on top.
02 - Combine apple cider vinegar, water, hot honey, granulated sugar, and kosher salt in a small saucepan. Bring to a simmer over medium heat, stirring constantly until the sugar and salt completely dissolve.
03 - Carefully pour the hot brine over the onions in the jar, ensuring all onions are fully submerged. Press down gently with a spoon if needed to release air pockets and submerge any floating pieces.
04 - Allow the jar to cool to room temperature, then cover with a lid and refrigerate for at least 1 hour before serving. For optimal flavor development, marinate overnight in the refrigerator.
05 - Store the pickled onions refrigerated for up to 2 weeks, ensuring the onions remain submerged in the brine.