Put the beef, soy sauce, sesame oil, Shaoxing rice wine or dry sherry and cornflour into a bowl and mix together until well combined. Set aside to marinate for 20 minutes.
Heat a wok or large frying-pan until it is very hot.
Add the oil, and when it is very hot and slightly smoking, add the beef slices and stir-fry for 5-6 minutes or until they are pale golden-brown.
Remove the beef strips and set aside to drain in a colander over a bowl. Discard the drained oil.
Wipe the wok or pan clean and reheat it over a high heat until it is hot.
Add the oyster sauce, and bring to a simmer.
Return the drained beef slices and mix them thoroughly with the sauce.
Turn the mixture onto a serving platter and garnish with the spring onions.
Meanwhile, for the salad, mix the mustard, curry powder, soy sauce, salt, pepper and olive oil together in a bowl.
Cook the broccoli, green beans and cauliflower in boiling water for 2-3 minutes, then add peas and cook for 1-2 minutes.
Drain the vegetables into a warm bowl, then add the water chestnuts and tomatoes, shallots and chives. Drizzle over the salad dressing and mix until well combined.
Serve the salad alongside the beef.