Put the noodles in a large saucepan of lightly salted boiling water. Return to the boil and simmer for 4 minutes, stirring occasionally with a fork. Drain, reserving 50 ml of the liquid. Rinse the noodles, then set aside.
Meanwhile, whisk together the stock, soy sauce, cornflour and sesame oil in a small bowl until smooth.
Heat the vegetable oil in a wok or large non-stick frying pan over a moderate heat. Reserve some of the dark parts of the spring onions for garnish, then add the remaining spring onions, garlic, zucchini and, if you are using it, chilli to the wok. Stir-fry for 3–4 minutes until softened.
Add the tofu, stock mixture and reserved noodle soaking liquid to the wok. Bring to the boil, stirring constantly for 1–2 minutes, or until the sauce thickens.
Add the noodles and toss together. Remove the wok from the heat, toss in the watercress, coriander and reserved spring onions and serve immediately.