Place beets in a large saucepan with enough cold water to cover by 1 inch (2.5 cm). Bring to a boil over high heat; reduce heat to medium-low, partially cover and cook beets until tender, 20 to 30 minutes. Drain and let cool. Peel and trim beets; cut crosswise into ¼-inch (5 mm) thick slices.
Cut peel and pith off oranges; thinly slice crosswise into ½-inch (1 cm) rounds, removing any seeds.
Poppy Seed Dressing
In a bowl, whisk yogurt, oil, vinegar, honey and poppy seeds; add salt and pepper to taste. Arrange overlapping sliced beets and oranges on a platter. Drizzle with dressing; sprinkle with mint and pistachios.
Per serving: 171 calories, 4 g protein, 7 g fat (1 g saturated fat), 24 g carbohydrates, 5 g fibre, 1 mg cholesterol, 132 mg sodium