Cut very thin julienne strips of zest from 1 orange and blanch in boiling water for a few minutes. Drain and reserve. Peel all the oranges, removing all the pith. With a sharp knife, cut down between the sections and remove them whole, free of membrane or seeds.
Cover an attractive plate with the watercress, reserving a handful of leaves for garnish. Pile the black olives in the center of the plate and arrange the orange sections around. Scatter the onion rings, julienne strips of orange zest, parsley, and celery leaves (if using) over the top.
Combine the olive oil, wine vinegar, and a seasoning of salt and pepper, and whisk all together. When ready to serve, spoon this dressing over the salad.
© 1999 Jennifer Paterson and Clarissa Dickson Wright estate. All rights reserved.