Pickled beets in the refrigerator

Bring vinegar, water, sugar, salt and mustard to a boil in a medium saucepan, stirring until sugar is dissolved. Simmer for a few minutes; remove from heat and let cool slightly.
Add peppercorns and beets to a clean glass jar with a tight-fitting lid. (This particular recipe fits nicely in a 1-quart jar.) Pour in the brine, covering all of the beets.
Cover and let sit for several hours (or up to 24), then transfer to the refrigerator.