Slice the red onions thinly. Dice the garlic. Put these in a glass container with a lid.
Combine the water, apple cider vinegar, salt, peppercorns, and hibiscus flowers (if using) in a small pot. Bring heat to medium and cook for 5 minutes stirring frequently.
Pour the warm liquid over the onions in the jar. Put the lid on and refrigerate for 3 hours.
After 3 hours, the pickled onions are ready to serve. Store any leftovers tightly sealed in the refrigerator.