01 - Pack the sliced red onions into a clean, heatproof 2-cup jar. Add the sliced garlic clove, red pepper flakes, and whole black peppercorns to the jar.
02 - In a small saucepan, combine apple cider vinegar, water, hot honey, granulated sugar, and kosher salt. Bring to a simmer over medium heat, stirring continuously until sugar and salt completely dissolve.
03 - Pour the hot brine over the onions in the jar, ensuring they're completely submerged. Press down with a spoon if necessary to fully cover all onion slices.
04 - Let cool to room temperature, then cover and refrigerate for at least 1 hour before serving. For optimal flavor development, marinate overnight.
05 - Store refrigerated for up to 2 weeks. Keep jar tightly sealed when not in use.