Slice the beef into stir fry strips.
Place the cornflour, Worcestershire sauce, tamari and sugar into a bowl big enough to marinate the beef, and whisk together. Add the beef and make sure it is well coated. Cover and leave at room temperature for up to an hour, or refrigerate if marinating any longer.
Follow the cooking directions for the Spaghettini pasta for al dente pasta, but subtract 2-3minutes from what they suggest. We will be cooking this again in a frying pan so we want to under-cook the pasta. Drain and rinse off the starch immediately. Set aside.
Heat some EVOO in a wok till very hot and fry off the onions for around 5 minutes. Add the mushrooms until softened, adding the beef strips and all the marinating liquid at the end. Cook a further 5 minutes before adding the drained noodles.
Heat through the noodles stirring to coat them with the sauce. Taste and add salt if you wish. Serve immediately with fresh herbs of choice and extra Worcestershire sauce on the side...that stuff is amazing!