In a pot, combine the beets with the red wine vinegar and enough cold water to cover them by 2 inches. Bring to a boil, lower the heat, and simmer until tender, about 40 minutes. Drain the beets and let them cool. (The beets may be cooked 1 day in advance and kept covered and chilled.) Peel the beets and cut them into 1-inch wedges.
In a small saucepan, combine zest, orange juice, sugar, and vinegar and boil the mixture until it is reduced to about 1/4 cup. Let the mixture cool.
In a large bowl, whisk together the orange mixture, oil, and salt and pepper to form a smooth dressing. Add the beet slices, orange segments, and onion and mix until well combined. Let stand at room temperature, or refrigerate for up to 6 hours, to let the flavors come together. Transfer to a serving bowl and garnish with the parsley.
