Start by slicing each chicken breast into 2 fillets, and season well with pepper and salt. Heat the oil in a griddle pan, and cook the chicken on both sides for about 4 minutes till golden brown. While the chicken is cooking make the walnut vinaigrette. Place all the ingredients together in a processor and pulse together to a creamy dressing. Place the rucola and mixed salad leaves in a bowl and add ½ of the walnut dressing. Mix well, and divide the salad leaves between 2 large salad plates. Arrange the remaining walnuts on top, along with the seedless grape halves. Place 2 pieces of the grilled chicken fillets on top of each salad, and drizzle over the remaining walnut vinaigrette. Serve sprinkled with parmesan shavings.