Rinse watercress and purslane. Drain. Cover and refrigerate until ready to use. Put broad beans in heatproof bowl and pour boiling water on top. Stand for 2 minutes. Drain. Pinch beans to remove outer skins and discard. Put beans in a separate bowl, cover and set aside.
To make dressing, put all ingredients, except oil, into a food processor. Add oil in thin, steady stream while processing until finely chopped and combined. Transfer dressing to jug.
Combine watercress and purslane on platter. Sprinkle beans, walnuts and parmesan over leaves. Tear prosciutto into smaller pieces and arrange on top. Drizzle dressing over salad and sprinkle with micro herbs.