Slice the red onion very thinly and set aside.
In a 2-cup jar, combine the apple cider vinegar, honey, salt, and mustard seeds (optional). Stir until well mixed.
Add the sliced onions to the jar, pressing them down to fully submerge them in the liquid. If needed, add 1 to 2 tablespoons more apple cider vinegar to ensure the onions are completely coated.
Seal the jar and flip it upside down a few times to ensure the onions are fully coated. Let the onions sit at room temperature for about 1 hour.
Once pickled, transfer the jar to the refrigerator to chill. The onions will be best enjoyed cold.