Lay the duck breasts, skin side down, on a board, cover with clingfilm or greaseproof paper and using a rolling pin, bash them to flatten them slightly. Remove the clingfilm, turn the breasts over and score the skin diagonally or in a crisscross pattern with a very sharp knife. Rub the skin all over with salt.
Put the duck breasts, skin side up, on a rack in a roasting pan and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 15–20 minutes or until the duck is brown on the outside but a little pink in the middle.
Meanwhile, cut the skin off the oranges and cut into segments, working over a bowl to catch the juice. Add the orange juice to the ingredients for the dressing, season to taste with salt and pepper and stir well.
Transfer the cooked duck breasts onto a chopping board and slice them thinly on the diagonal. Put the chopped rocket and watercress leaves in the centre of 2 serving plates and arrange the duck slices and orange segments on top.
Whisk the dressing and spoon it over the salad.