Put your couscous in a bowl and pour over just enough boiling water or chicken stock to cover it. Set aside for a couple of minutes to allow the couscous to soak up in the water. In the meantime whiz all the dressing ingredients. Cut the chicken in thin slices and combine them with the rest of the ingredients in a bowl: the couscous, peppers, onion, corn, white mushrooms and parsley. Spoon the dressing over, give everything a good toss and divide between four different bowls.