Place purple onions and peppercorns into a medium-sized non-reactive bowl. Sprinkle with 1-2 teaspoons salt; stir and pound lightly with a wooden spoon. Cover bowl with a towel and allow onions to sit undisturbed for 10-15 minutes.
Scoop the salted onion mixture into two clean pint jars, leaving about an inch of space from the top of the jar. Press relish down with a wooden spoon to compact it and cause the juices to rise.
Fill remaining space in the jars with filtered water, leaving 1 inch of headspace. Add a teaspoon of starter, if desired.
Cover jars with lids (you can use an airlock or a regular lid) and allow to sit at room temperature away from drafts and direct sunlight for 4-5 days. Burp jars regularly and press the relish down into the liquid daily to keep onions submerged.
Once the relish has become bubbly and fragrant, place the jars in cold storage until you are ready to use them.
This relish will keep sealed in the refrigerator for 6 to 7 weeks.