Step 1

Bring a small saucepan of salted water to a boil. Add onions, and boil until partly tender, about 2 minutes. Drain and rinse several times with cold water. Transfer onions to a pint-sized jar.

Step 2

In a small saucepan, bring the water, vinegar, sugar, mustard seeds, coriander seeds, and black peppercorn mixture to a boil. Pour mixture over onions, place lid on top, and refrigerate at least overnight.