Preheat the oven to 350°.
Slice off the stem end of the beets. Place them in a pan or pot cut side down along with 1 cup of cold water.
Cover with a lid or foil and roast in the oven for 1 hour at 350°.
Remove them from the pot and let them cool for about 30 minutes on a plate or platter.
Using gloves, clean hands, or paper towels, remove the outside peelings of the beets.
Cut the beets into thick slices or wedges.
Place the cut beets into clean and sterilized jars.
Next, bring the water, vinegar, sugar, salt, mustard seeds, and peppercorns to a boil in a large pot over high heat. Make sure the salt and sugar are completely dissolved before removing it from the burner.
Pour the brine over the beets in the jar making sure to submerge them.
Let cool to room temperature before wiping the rim of the jar dry, adding on the seal and lid, and giving a firm twist using only your fingertips. Place them in the refrigerator to completely cool before eating.
To optionally can these beets for long-term storage, add the sealed jars to a large pot of boiling water with a rack in the bottom, making sure they are submerged by one inch. Process them for 30 minutes at sea level. Remove them from the water bath and cool to room temperature. Store in a cool, dark place for up to 6 months.
