In a covered large saucepan, cook medium beets in boiling salted water for 35 to 45 minutes or until tender; drain. Cool; slip off skins. Using an apple corer, cut a hole through the center of each medium beet. If necessary, cut a thin slice off the bottom of each beet so it sits upright. Cover and chill for 1 to 24 hours.
Meanwhile, in a covered medium saucepan, cook baby beets in boiling salted water for 8 to 10 minutes or until tender; drain. Cool; slip off skins and halve the beets. Cover and chill for 1 to 24 hours.
For dressing, in a screw-top jar, combine orange juice, olive oil, balsamic vinegar, shallot, Dijon mustard, and salt. Cover and shake well.
To serve, place a medium beet upright on each salad plate. Place 1 cup of the salad greens in the center and around each medium beet. Arrange four baby beet halves around each beet. Drizzle with dressing. Sprinkle with pepper. Makes 8 servings.