First of all, tear the lettuce and chicory leaves and place them in a large serving bowl, along with the rocket and watercress.
Now scatter in the cucumber and spring onions and toss well. Next, make the vinaigrette. Begin by crushing the salt quite coarsely with a pestle and mortar, and then add the garlic. As you begin to crush it and it comes into contact with the salt, it will quickly break down into a purée. Next, add the mustard powder and really work it in, giving it about 20 seconds of circular movements to get it thoroughly blended. After that, add some freshly milled black pepper. Now add the vinegars and work these in in the same way.
Then add the oil, switch to a small whisk and give everything a really good whisking. as for the potato salad, above. Just before serving, whisk the dressing again, add half of it to the salad leaves and toss them, then add the remaining dressing and toss again, so that everything gets a good coating. Serve immediately.