Preheat the oven to 400°F.

Place each beet on a piece of foil. Drizzle with olive oil, sprinkle with salt, and rub with your hands to coat. Wrap tightly in the foil and place on a baking sheet.

Roast for 40 to 60 minutes, or until fork-tender. The timing will depend on the size and freshness of the beets.

Remove the foil and set the beets aside to cool. When cool enough to handle, use your hands to slip off the skins (I like to do this under cool running water). Slice the beets into wedges and place in a (32-ounce) or similar-size glass jar with a lid.

Heat the water, apple cider vinegar, sugar, and salt in a small saucepan over medium heat. Cook, stirring, for 1 to 2 minutes, or until the sugar and salt dissolve. Remove from the heat.

Add the garlic, mustard seeds, and peppercorns to the jar with the beets and pour the brine on top.

Set aside to cool to room temperature, then cover the beets and store in the fridge. They will be ready to eat after 1 day and keep for up to 2 weeks.