Mix all marinade ingredients together. Add the beef and leave for 15 minutes.
Heat the wok. Add oil and stir-fry the pepper, sweetcorn, mangetout and scallions. When cooked remove from the wok. Now stir-fry the beef in batches, with extra oil if necessary. Remove from the wok and hold with the vegetables. Return the beef and vegetables to the wok and heat. Finish with a dash of sesame oil.
Blanch the spinach in boiling water for 2-3 minutes. Drain well. Toss in a little sesame oil. Place on a serving dish. Spoon the beef and vegetable mixture onto the hot spinach and serve immediately.