Cut the leaves from the stems of the bok choy.  Trim the stems as necessary and roughly chop into 1 inch pieces.  Roughly chop the leaves.
Put the oil into a large skillet and heat over medium high heat.  Once the skillet is hot, add the stems and cook, stirring occasionally, until they just lose their crunch, about 3 minutes.  Add the greens and add about ½ cup water or stock.
Cook, stirring occasionally, until the liquid evaporates and the stems become very tender, about 10 minutes.  Sprinkle with salt and pepper and serve immediately.