Using a sharp knife, cut the potatoes in half and peel some skin off, leaving a band in the center for color.  Trim off the ends of the cucumber and remove the skin. Cut the cucumbers into 2-inch rounds. Quarter each slice lengthwise and remove the seeds. Put the potatoes in the rack of a steamer over boiling water and steam for 10 minutes. Add the cucumbers and steam for 3 minutes more. Do not overcook.  Heat the butter in a saucepan and add the potatoes, cucumbers, parsley, salt and pepper. Add the lemon juice and toss. Heat for 30 seconds.