Preheat oven to 400°.
Steam first 3 ingredients, covered, 5 minutes. Place in a shallow roasting pan coated with cooking spray.
Combine syrup and vinegar; set aside.
Squeeze juice from lemon into a bowl; add lemon halves to beet mixture. Combine juice, oil, coriander, tarragon, and 12 thyme sprigs. Pour over beet mixture; toss well. Sprinkle with salt and pepper. Bake at 400° for 30 minutes. Pour syrup mixture over beet mixture; stir well to coat. Bake an additional 30 minutes or until beets are tender. Discard lemon halves. Garnish with additional thyme sprigs, if desired.