Step 1: Trim the beet greens, leaving about 1 inch of the stem and the root intact to prevent bleeding. Scrub the beets thoroughly under cold water.
Step 2: Place the beets in a large pot and cover with water. Bring to a boil, then reduce heat and simmer for 30-45 minutes until the beets are fork-tender.
Step 3: Drain the beets and immediately plunge them into a bowl of ice water. Once cool enough to handle, use your fingers to slip the skins off. Trim the remaining stems and roots.
Step 4: Slice the peeled beets into 1/4-inch thick rounds or wedges. If using onions, layer the sliced beets and onions into clean glass jars.
Step 5: In a medium saucepan, combine the apple cider vinegar, sugar, water, salt, cinnamon stick, cloves, and allspice. Bring the mixture to a boil, stirring to dissolve the sugar. Simmer for 5-10 minutes.
Step 6: Pour the hot brine over the beets in the jars, ensuring they are completely submerged. Allow to cool to room temperature before sealing.
Step 7: Refrigerate for at least 24 hours before serving to allow the flavors to fully develop. These will stay fresh in the refrigerator for up to 1 month.