Preheat the oven to 400 degrees.
Remove the tops and the roots of the beets and peel each one with a vegetable peeler. Cut the beets in 1 1/2-inch chunks. (Small beets can be halved, medium ones cut in quarters, and large beets cut in eighths.)
Place the cut beets on a baking sheet and toss with the olive oil, thyme leaves, salt, and pepper. Roast for 35 to 40 minutes, turning once or twice with a spatula, until the beets are tender.
Remove from the oven and immediately toss with the vinegar and orange juice.
Sprinkle with salt and pepper and serve warm.