Trim 6 medium beets; reserve the greens. Rub the beets with olive oil; season with salt and pepper. Wrap individually in foil and roast at 350 degrees F until tender, 1 to 1 1/2 hours; let cool. Peel and cut into wedges. Cook the beet greens in boiling water, 3 minutes; drain, squeeze out the excess water and chop. Toss the beets and greens with 1/2 cup sliced red onion, 1/4 cup chopped chives, 5 tablespoons olive oil and 3 tablespoons red wine vinegar. Season with salt and pepper.