Cover beets in water and simmer in a covered saucepan for about 30 minutes. When easily pierced by a fork, they are ready. Drain and rinse in cold water. When cool, slip off skins. Slice beets ¼" thick.
To make the dressing, add scallions, vinegar, lemon juice, mint, zest, salt and pepper in a bowl. Add oil slowly, whisking until blended. Add beets and toss. Add more vinegar or salt as needed.