Preheat the oven to 400°F/200°C. Trim the beets and scrub them well, removing any debris or rough spots. Rinse well. Reserve any nice leaves and wash in cold water. Spin the leaves dry and then finely slice them. Cut the beets into quarters, or halves if small.Transfer to a sheet pan, toss with enough olive oil to coat (about 2 tablespoons), and sprinkle generously with salt and pepper.Roast until the beets are tender when pierced with a fork, 45 minutes to 55 minutes.Combine the vinegar, onions, and garlic in a medium bowl. Stir in the hot beets, sliced leaves (if using), and the mint leaves; toss well. Season with salt and pepper as needed.Leave to marinate in the refrigerator for at least 2 hours, preferably overnight, and stir a few times while they're marinating. Serve at room temperature with fresh mint leaves on top.