Heat the grill. Brush the chicken with the oil and sprinkle with a little salt and pepper. Grill for about 5 minutes on each side. To test if the chicken is cooked -insert a skewer or knife into the chicken - the juices should run clear with no trace of pink when the chicken is cooked.
Remove from the grill and allow to cool. Cut the chicken breasts into thick slices.
Dissolve the stock cube in the boiling water in a pan over a low heat. Gradually stir in the couscous and bring to a boil. Remove from the heat, cover the pan with a well fitting lid and leave to stand for 1 minute.
Remove the lid and, using 2 forks, fluff up the couscous grains. Add the herbs, tomatoes, onion and scallions and mix well. Season to taste with salt and pepper.
Spoon into serving bowls and arrange the sliced chicken on top.