Mix the beat root, most of the lemon juice, 2 Tbsp. of the olive oil with salt and pepper in a bowl. Cover and set aside. Peel the oranges and remove any white pith. Cut crossways into ¼ in slices. Remove the pithy center with the point of a knife. Spread the slices on a plate, cover tightly and refrigerate until chilled.

To serve, line the edges of a chilled serving platter with the watercress and lay the orange slices in a single overlapping layer on the inside edge of this border. Make a circle of beet root inside the oranges and drizzle the marinade over them. Place the feta in the center of the platter.
Sprinkle the oranges with a little salt, and the beet root and cheese with the rigani and olives. Sprinkle the remaining lemon juice, olive oil and pepper over the salad and serve at once.