Thoroughly wash the beets, and trim off the tough top and bottom end. You can leave them unpeeled, or peel the beets if you wish.
Cut the beets into equal size pieces, such as halves, quarters, or bite-size chunks. Put them in a deep roasting pan or casserole dish.
Add the fresh-squeezed juice from one orange, a drizzle of olive oil, and generous drizzle of balsamic vinegar (enough to create a shallow pool of marinade across the bottom of the dish). Top with a sprinkle of sea salt, optional black pepper and orange slices.
Bake on 400F until fork tender, about 45 minutes on average. Cover the pan for the first half with a lid or foil, and then leave uncovered during the second half. Toss/stir halfway through. Smaller pieces will cook faster than larger chunks.
Once finished, transfer the beets AND the marinade juices into a glass food storage container with a lid, and store in the refrigerator for up to a week.