01 -
Combine apple cider vinegar, honey, red pepper flakes, salt, and black pepper in a medium saucepan.
02 -
Warm the mixture over medium heat, stirring occasionally, until it begins to simmer.
03 -
Arrange the sliced red onions in a clean glass jar or nonreactive heatproof container.
04 -
Carefully pour the hot vinegar mixture over the onions, ensuring they are completely submerged.
05 -
Allow the contents to cool to room temperature, then seal tightly and refrigerate.
06 -
Let the onions pickle for a minimum of 24 hours before serving for optimal flavor development.