In a pot, add the vinegar, water, sea salt, sugar and peppercorns. If using any other additional seasonings, add them now too. Stir together. Bring to a boil for 2 minutes.
Add in the onions, and allow to boil for 1 minute with the onions.
Remove from heat, let cool for 1 minute, then pour into a mason jar large enough to hold everything but that allowed the onions to be completely covered in the vinegar mixture. Add in the lime slices and swirl together to mix everything together.
Allow the mixture to cool completely to room temperature. You can serve immediately once cooled, but if you let it sit and continue to pickle, the flavours meld incredibly. I tried mine three days later and they were perfect. A week is even better. They will last several weeks in your fridge in a mason jar that has the cover tightly screwed on.