First make the dressing. Put the olive oil, balsamic vinegar, mustard, sugar and salt and pepper to taste in a small screw-top jar. Screw on the lid and shake all together until well blended. Keep in the refrigerator until needed.
Preheat the grill to high. Place a strip of foil on the grill tray.
Using a vegetable peeler, peel the pecorino cheese into fine shavings. Reserve half of these and finely chop the remainder. Put the watercress, rocket and grapes in a salad bowl and toss together.
Peel, halve and core the pears. Arrange the pear halves, cut-side down, on the foil strip. Top the pears with the shavings of cheese, slightly overlapping them. Place under the grill and grill for 2 minutes or until the cheese just starts to bubble and turn golden.
Meanwhile, shake the dressing, pour it over the salad and toss to coat the leaves. Add the chopped pecorino. Divide the salad equally among two or four plates, then top with the pears.