Dress the pears with the lemon juice. Place the walnuts in a small frying pan over low heat and cook until golden. Set aside to cool slightly.
To make the dressing, combine the olive oil and vinegar in a bowl. Season the dressing with salt and cracked black pepper, then whisk until well combined.
To serve, combine the rocket, pear, walnuts and dressing in a large bowl and gently toss to combine. Transfer to a serving bowl and top with shaved parmesan.