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Cook the peas in a large pan of boiling salted water for about 3 mins (or 1 min if frozen). Drain and run under cold water to stop them cooking. Whisk the dressing ingredients together with some seasoning until emulsified.
Mix the ham hock with the peas, parsley and dressing in a large bowl. Transfer the salad to a platter or spoon onto individual plates. Scatter over the spring onions and watercress just before serving.