Step 1: Trim the beet greens and roots, leaving about an inch of the stem attached. Wash the beets thoroughly.
Step 2: Place the whole, unpeeled beets in a large pot and cover with water. Bring to a boil, then reduce heat and simmer until fork-tender (30-60 minutes, depending on size).
Step 3: Drain the cooked beets and let them cool slightly. Peel the skins off under cool running water (wear gloves to avoid staining) and then slice them into 1/4-inch thick rounds, half-moons, or wedges.
Step 4: In a medium saucepan, combine apple cider vinegar, water, sugar, salt, peppercorns, mustard seeds, cloves, and optional bay leaves. Bring to a boil over medium-high heat, stirring until sugar and salt are dissolved.
Step 5: Pack the sliced beets snugly into four clean, pint-sized heat-proof jars, leaving 1/2-inch headspace.
Step 6: Carefully pour the hot brine over the beets, ensuring they are fully submerged. Distribute the spices evenly among the jars. Tap jars gently to release air bubbles.
Step 7: Seal the jars tightly with lids. Let them cool completely at room temperature, then refrigerate. For best flavor, allow to marinate for at least 24 hours, ideally 2-3 days, before serving. Enjoy within 2-3 weeks.