Refrigerate cooked rice until cold. Peel the avacado and slice lengthwise into 1/2 inch-thick strips. Coat with lemon juice, and refrigerate until well chilled. In a large mixing bowl, combine the scallions, olives, chicken and rice. Toss gently just until mixed. In small bowl, mix vinegar, mustard, veg. oil, sugar, and parsley. Whisk together vigorously until completely combined. Just before serving, add the avocado slices to the chicken and rice mixture. Pour on dressing and toss gently to combine thoroughly. Serve sprinkled with toasted nuts and cherry tomatoes.