Slice the red onions thinly.
In a bowl, combine vinegar and water.
Add sugar and salt to the vinegar mixture and stir until dissolved.
Place the sliced onions in a jar or airtight container.
Pour the vinegar mixture over the onions, ensuring they are fully submerged.
Add any optional spices for additional flavor.
Seal the jar tightly and shake gently to combine.
Let it sit at room temperature for at least 1 hour.
For best flavor, refrigerate for 24 hours before using.