In a saucepan, bring the vinegar mixture, sugar, onion, garlic, mustard seeds, bay leaves, cloves, peppercorns, thyme and oregano to a boil. In a large pot of boiling salted water, cook the beets until crisp-tender, about 10 minutes. Drain and transfer to a heatproof bowl. Pour the hot liquid over the beets and let stand at room temperature for 4 hours or refrigerate overnight. Drain the beets, transfer to a bowl and serve.