Pak Choi is also called Bok Choy in America and is a a leafy green that is a type of Chinese cabbage.
The texture of both leaves and stalks is crisp, and the flavour is somewhere between mild cabbage and spinach. Its also one of the quickest vegetables to cook. If very young it can be eaten raw in salads, but is best when briefly cooked.
There are three main cooking methods for pak choi - Stir-fry (2 mins), steam (sliced 2-3 mins; whole up to 8 mins) and simmer in water (2-3 mins). Be careful not to overcook or you will lose that refreshing crunch from the juicy stalk which makes a great textural contrast. Pak Choi cooks down quite a bit, so the raw amount may look like it will be too much before cooking.
Pak Choi pairs well with simple addition of flavours - Garlic, Ginger, Soy sauce, Oyster Sauce or Sesame oil.