Bake in the oven for about 18 to 20 minutes, or until desired doneness. I find that 20 minutes gives you nice, crisp edges. Alternatively, you can grill: using a medium-hot fire in a charcoal or gas grill: Arrange the bok choy, cut side down, on the grill grate. (If you have a grill screen or tray, set it on top of the grate before adding the bok choy, to keep the stalks from falling through the gaps.) Close the lid and grill for about 5 minutes, until golden brown on the underside. Flip the bok choy with tongs, close the grill again, and grill for about 5 to 6 minutes more, until golden and crisp-tender.