Wash the potatoes, then cook them in lightly salted water.
Meanwhile, chop the walnuts and roast them in a dry pan. Let them cool.
Separate the chicory leaves, then slice them into long, thin strips (lengthwise).
Slice the spring onions diagonally.
Mix the dressing, taste it and adjust the acidity with some lemon juice.
Drain the potatoes and mix them immediately with the dressing.
Carefully fold in the chicory strips, spring onion and walnut pieces, then crumble the cheese on top.
Stir in the watercress just before serving.