Heat the oven to 425°F/220°C. Chop the tops off the beets, leaving about an inch/2.5 cm of green on the bulb, and set aside. Remove the tail-like root end of the beet and discard. Cut each beet into about eight sections (or halve small beets), as you would cut an apple for pie, leaving a little green on the top of each wedge. Put the beets on a baking sheet. Drizzle over the olive oil and toss the beets to coat well. Dot with butter and season with salt and pepper. Roast until they are soft and well caramelized, about an hour, depending on the size of the beets (small beets will obviously cook faster).
Just before the beetroot is done, heat a little more olive oil in a sauté pan. Chop the green beet tops along with the red stems, and sauté until tender, about 5 minutes. Season with salt and pepper. Serve with the roasted beets.