Combine the dashi, mirin and shoyu in a pan over medium heat and bring to the boil. Remove from the heat and set aside to cool. Roast the pine nuts in a dry pan over medium heat until brown. Set aside to cool.
Wash and trim the watercress, then boil in a large pan with plenty of lightly salted water for 2 minutes. Refresh in cold water and squeeze out any excess moisture. Cut into 2.5 cm (1 in) pieces, pour over the sauce and refrigerate for at least 30 minutes. Serve with roasted pine nuts on top.