Combine the dressing ingredients and set aside. Heat a char grill pan and sear the beef fillet for a couple of minutes each side until it's brown. Wrap it in foil and set aside for around 10 minutes. Slice finely.
Bring a pan of slightly salted water to the boil and add the noodles. Cook for approx 4 - 5 minutes until they are soft. Drain and rinse in cold water.
Heat a wok with the remaining tea oil and stir fry the bok choy. Remove fro the heat and add the noodles, carrot, shallots, coriander, and beef. Toss together with the dressing and serve.
Note: Buckwheat soba noodles are available from supermarkets, health food stores and Asian grocers. If there are a number of brands available, check the ingredient list and select the with more buckwheat than wheat flour.