Trim and wash the beets and boil for 30 minutes, or until tender. After that, plunge them into cold water and slough off outer skin. Set the beets aside and allow them to cool completely. Next, slice the onions into rings and separate. Pour 1 tablespoon olive oil in a saucepan over medium heat and add the onions. Cook them until they are tender but still vibrantly colored. Swap the onions with the green beans and cook until tender.