Cook beets following the directions below in the notes section. Allow beets to cool enough to handle. Slip the skins off under cool running water. Cut into wedges.
Meanwhile, make the pickling liquid. Add the vinegar, water, sugar, salt, peppercorns, mustard seeds and red pepper flakes into a small saucepan set over medium heat.
Place the beet wedges, garlic, and dill into a quart sized canning jar, leaving at least 1/2" to the top of the jar. Pour the pickling liquid over the beets into the jar, leaving 1/2" headspace.