Wash and place beets on a sheet of tin foil, sprinkle with olive oil and sea salt. Wrap foil around beets and roast in the oven for 1 hour at 400 degrees. Let cool and peel beet skins, place in bowl and refrigerate overnight.
Just before serving, place lettuces in bowl. Slice beets into medium slivers or slices and place over lettuce. Scatter feta cheese on top and garnish with mandarin orange and cucumber slices. Pour dressing over and serve with reserve dressing on the side.
Place oil in bowl, squeeze juice from lemon and orange into oil, add vinegar, mustard, chopped onions, honey, salt & pepper (to taste) and whisk until incorporated. Pour over salad immediately, or if done ahead, re whisk prior to serving.