Cook pasta according to package directions, adding asparagus in the last 2 minutes of cooking time. Drain and refresh under cold running water; drain well and set aside.
In food processor, pulse 1/4 cup (60 mL) walnuts and garlic until finely ground. Add watercress, half of the parsley, olive oil, lemon zest, salt and pepper; pulse until smooth. Stir in Pecorino and Parmesan until blended.
In bowl, toss together pasta, asparagus and watercress pesto. Serve at room temperature or refrigerate for 1 to 2 hours to serve chilled.
Just before serving, garnish with shaved Parmesan, and remaining walnuts and parsley.