Put 1tbsp of the soy sauce in a small bowl and stir in the cornflour until smooth. Add the remaining soy sauce and 150ml cold water and stir well; set aside. Trim the pak choi and shred thickly. Separate the pale green stems from the leafy parts and set both aside. Trim the beef of any hard fat and cut into thin strips from one short end, diagonally along the length of the beef. The strips should be roughly 5mm thick. Put in a bowl and toss with 1 tsp of the oil. Season well with ground black pepper.
Place a large non-stick frying pan or wok over a high heat and when hot add the beef and stir fry for 1 minute until lightly browned. Tip immediately onto a plate and return the pan to the heat. Add the remaining 2 tsp of oil and the mushrooms, carrots, onion, pepper and pale green pak choi stems. Stir fry over a medium-high heat for 3 minutes until just beginning to soften. Add the garlic and ginger and stir fry for 1 minute more.
Return the beef to the pan and add the pak choi leaves. Stir the cornflour mixture and pour over the beef and vegetables. Cook together for 1-2 minutes until the beef is hot and the sauce is thickened and glossy. Serve with freshly boiled rice.
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