Remove beets from greens, leaving 1″ of stems. Wash beets under cool running water, then steam until easily pierced with a fork (20-35 minutes, depending on size). Rinse under cool water once done, and slip off skins. While the roots are steaming, wash the greens and saute them with a little olive oil until stems are tender. Slice beets and toss with balsamic vinaigrette and crumbled feta, and serve over warm beet greens.