Place the beets in a baking dish, and bake at 425° for 1 hour and 10 minutes or until tender. Cool beets. Combine 2 tablespoons orange juice, 1 tablespoon lime juice, and 1/8 teaspoon pepper. Peel beets; cut each into 8 wedges. Toss beets with orange juice mixture.
Combine 2 tablespoons orange juice, 1 tablespoon lime juice, 1/8 teaspoon pepper, mustard, oil, and salt. Combine the salad greens and diced mango. Drizzle with mustard mixture, and toss well to coat. Divide salad evenly among 4 plates, and top with beet wedges.