Preheat the oven to 350 degrees. Wash and peel the beets. Slice each beet in quarters lengthwise, and then slice each quarter into three chunks.
Line a baking sheet with foil, and place the beets on top. Drizzle with the olive oil and vinegar, and then add salt and pepper. Gently stir to coat the beets.
Cover with more foil and roll the edges of the two layers to create a pouch for the beets. Cook for 20 minutes.
Remove the beets from the oven, and raise the temperature to 400 degrees. Take off the top layer of foil (watch out for steam!), and stir to mix the juices. Return to the oven and continue to cook for 35 minutes, stirring occasionally.