Cut away and discard the stem and root ends of the beets. Wash and scrub very well. Place the beets in a medium saucepan in ample salted water and simmer for about 50 minutes to 1 hour, or until tender. Remove, drain and rinse in a colander under cold water. When the beets are cool enough to handle, peel them.

Using a sharp knife, cut the beets into very thin rounds. Cut the onion half into very thin slices.

Place about a quarter of the beets in a shallow serving bowl. Season with salt and pepper and sprinkle lightly with the fine zest. Strew about a quarter of the onion slices over them. Drizzle with 1 tablespoon of olive oil and ½ teaspoon of sherry vinegar. Repeat with remaining ingredients, finishing with a layer of onion slices. Garnish with fresh mint and serve.