Use only young tender spinach for this salad. Outside leaves are too bitter and tough to be enjoyed raw. Wash the spinach thoroughly, discarding any discoloured leaves and tough stalks. Tear into manageable pieces.
Wash the watercress, discarding tough stalks and yellow leaves. Shake water off the spinach and watercress in a lettuce basket.
Peel the orange. Remove the pith and pips and slice as finely as possible. Cut the slices into quarters.
Toss the ingredients together in a large bowl with the oil and season well.