Slice the ends off of the red onion, peel, and then slice in half. Then, slice the onion into thin using a mandolin slicer. I use a cut resistant glove to stay safe doing this! Add the onions to a fine mesh strainer and set aside.

Bring a pot of water to boil. Add the onion slices to a fine mesh strainer over the sink and carefully pour the boiling water over the onions to soften. Allow the water to fully drain. 

Carefully add the onion slices to a jar (I use this weck jar) and add peppercorns and salt. Pour apple cider vinegar over the onions and push down to fully submerge. 

Allow the jar to sit at room temperature for about half an hour, then transfer to the fridge. Store for up to 2 weeks in the fridge and serve over salads, toast, etc!