Fill tea kettle water and bring to a boil.

In a 8 ounce mason jar (half-pint), mix together vinegar, water, sugar, salt, and peppercorns.

Peel the onion, leaving the root end in tact (this will give you a better grip). Using a mandolin, thinly slice the the onion. Place in colander.

With the onion filled colander in the sink, slowly pour the boiling water over the onions. Drain well, and then add them to the mason jar with the marinade inside.

Mix everything together, cover, and rest at room temperature (or in the fridge) for several hours. Keep stored in the refrigerator. Will keep "indefinitely", according to the cookbook recipe notes.