1. Bring the vinegar, water, sugar, salt, and mustard to a boil in a medium saucepan, stirring until sugar is dissolved. Simmer for a few minutes; take off the heat and allow to cool a bit.
2. Add peppercorns and beets to a clean glass jar with a tight fitting lid. (This particular recipe fits nicely into a 1 liter sized jar.) Pour the brine in, covering all the beets.
3. Cover and set aside for several hours (or up to 24), then transfer to the refrigerator.

Credit: recipesneed