Combine first 3 ingredients; place in a single layer on a baking sheet.
Bake at 275° for 40 to 45 minutes, stirring every 15 minutes. Cool and set aside.
Cut tops from beets; slice tops, and set aside. Peel beets, and cut into wedges.
Toss together beet wedges, olive oil, salt, and pepper; place in a 15- x 10-inch jellyroll pan.
Bake at 400°, stirring occasionally, 35 to 45 minutes or until tender. Toss with orange sections and half of Orange Vinaigrette.
Combine mixed salad greens and sliced beet tops; toss with remaining dressing. Arrange on individual serving plates; top with beets and oranges, and sprinkle with walnuts.