Heat the wok for 45 seconds to 1 minute, pour oil into the wok, and coat the sides by spreading oil with a spatula. Drop a slice of ginger into the oil; when it becomes light brown, the oil is ready. (When cooking vegetables I usually add a little salt to the oil, but not when cooking meat or fish, which usually have been marinated or otherwise preseasoned.) Then place the food in the wok and begin tossing it through the oil, 1 or 2 minutes for such soft vegetables as bok choy or scallions, about a minute longer for harder vegetables such as cabbage, carrots, or broccoli. Scoop out the vegetables with a spatula and they are ready to be served.