Preheat oven to 400°F. Trim beet stems, leaving about 1 inch. Wrap each beet individually in foil. Place on a baking sheet and roast for 45-60 minutes until tender when pierced with a fork. Allow to cool completely.

Once cooled, use your hands or paper towels to gently rub off the beet skins. They should slip off easily. Slice the peeled beets into 1/4-inch thick rounds or wedges.

In a medium saucepan, combine apple cider vinegar, water, sugar, salt, and ground mustard. Bring the mixture to a boil over medium-high heat, stirring until sugar and salt completely dissolve. Simmer for 2-3 minutes, then remove from heat and let cool slightly.

Place the sliced beets and peppercorns into a clean glass jar with a tight-fitting lid. Pour the warm brine over the beets, ensuring they are completely submerged.

Allow the jar to cool to room temperature with the lid loosely placed on top. Once cooled, tighten the lid and refrigerate. The pickled beets can be eaten after a few hours but taste best after 24-48 hours.