Preheat oven to 350°F. Slice off beet stems and place beets cut side down in a pot with 1 cup water.
Cover the pot and roast beets in the oven for 1 hour until tender.
Remove from oven and let cool for 30 minutes until safe to handle.
Peel the cooled beets and slice into desired thickness.
Pack the sliced beets into sterilized jars.
In a saucepan, combine 1½ cups water, vinegar, sugar, salt, mustard seeds, and peppercorns.
Bring the brine mixture to a boil, stirring until salt and sugar completely dissolve.
Pour the hot brine over the beets in jars, ensuring they are completely submerged.
Allow jars to cool to room temperature, then seal and refrigerate.
For long-term storage, process sealed jars in a boiling water bath for 30 minutes.