Heat the oven to 425 degrees F/220 degrees C.
Chop the tops off the beets, leaving about 1 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 some olive oil and toss the beets to coat well. Dot with some butter and sprinkle with some salt and pepper. Roast until they are soft and well caramelized, about 1 hour, depending on the size of the beets (small beets will obviously cook faster).
Just before the beetroots are done, heat a little more olive oil in a saute pan. Roughly chop the green beet tops along with the red stems, and saute until tender, about 5 minutes. Season with salt and pepper. Serve tossed with the roasted beets.