Cook the asparagus in boiling salted water until tender - approximately 3-4 minutes then drain. Cook the peas in the same way, then drain and reserve.
Chop the mozzarella into bite-size pieces and arrange all together with the watercress, rocket and spinach. Whisk the dressing ingredients together with a little salt and pepper.
Drizzle over the salad and toss thoroughly just before serving.