To make the onions, bring 2 to 3 cups of water to a boil.
In an airtight container, add salt, apple cider vinegar, and peppercorns. Stir to dissolve the salt. Set aside.
Place onions in a sieve over the sink and slowly pour boiling water over the onions. Let them drain. Add onions to the airtight container. Stir to evenly distribute onion. Let stand 30 minutes, up to a handful of hours, before using.