Place purple onions and peppercorns into a medium non-reactive bowl. Sprinkle with 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 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.
Cover with an airlock fermentation lid or regular, closed tightly.
Ferment 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, move the jars to cold storage.
This relish will keep sealed in the refrigerator for 6 to 7 weeks.