Oil a foil-lined baking sheet and scatter the Parmesan over it, spreading it into a thin layer about 25 cm (10 inches) square. Cook under a hot grill for 2–3 minutes until the cheese has melted and is pale golden in colour. Leave until cool enough to handle, then peel the foil away, letting the cheese break into pieces to form 'croutes'.
Whisk together the dressing ingredients in a large bowl. Halve and core the pears then cut into thin slices.
Add the pears, walnuts and salad leaves to the bowl with the dressing and toss together. Pile on to 4 serving plates and scatter with the Parmesan croutes.