Mix the beetroot with most of the lemon juice, 2 tbsps of the olive oil and the seasoning in a bowl. Cover and set aside.
Peel the oranges and remove any white pith. Cut crossways into 1/4 ” slices. Remove the pithy centres with the point of a knife. Spread the slices on a plate, cover tightly and refrigerate until chilled.
When you are ready to serve line the edges of a serving platter with watercress and lay the oranges in a single overlapping layer on the inside edge of the watercress. Make a circle of the beetroot inside the oranges and drizzle over the marinade.
Put the feta in the centre. Sprinkle the oranges with a little salt and the beets and the cheese with the rigani and olives.
Sprinkle the remaining lemon juice, olive oil and pepper over the salad and serve immediately.