In 2 separate pots, cook your beets in water for 1 to 1 and half hour depending on the size. Put your knife through it, it should feel like soft.
Allow to cool. In the meantime, wash your apples.
Cut beets and apples in horizontal thin slices. Build you 'tower' by alternating the yellow beet, red beet and apple.
Sprinkle with feta.
Vinaigrette: Mix all the ingredients together.