Wash and place the beetroot in a pot with plenty of water. Bring to a boil, lower the heat and cook for 30-40 minutes until tender. Cool under plenty of cold running water and remove the skins. Dice into 2 1/2 cm (1in) pieces. Place in a bowl and add a pinch of salt.
Halve the oranges from top to bottom and cut into 1/2 cm (1/4 in) slices. Add the oranges and the walnuts to the beetroot.
In a small bowl mix the olive oil and the orange juice.
Pour over the beetroot, orange and walnut halves and toss well. Check seasoning. Lightly toss in the watercress. Arrange on plates and serve.