Bok choy, a type of Asian cabbage, is sold in most supermarkets. It has a dense, chewy rib attached to a more delicate leaf, and is usually sold in both mature and baby forms, ranging from about the size of a fist to about the size of a football. Because bok choy includes a leafy green as well as a solid stem, substitutes should either have leaves and stems as well, or you should replace bok choy with two different vegetables, one sturdy and the other leafy.
Bok choy is a common stir-fry ingredient, and it is usually cooked with a combination of other vegetables. Napa cabbage makes a natural substitute because it is a close relative and it contains both leaf and stem. Napa is paler green, so you may need to change your other vegetable choices to get the same vibrant result.
Baby bok choy blends well with other leafy greens in mixed green salads, and it can be replaced with any dark green leaf or any cabbage with a fairly neutral flavor. Tatsoi is similar, but Swiss chard can stand in as well, especially because its stem has a similar texture to a bok choy stem. Napa cabbage or broccoli make suitable substitutes in noodle salads, where the bok choy serves to add crunchiness.
On the Grill
Grilled bok choy makes a light, flavorful accompaniment to steaks and burgers. It can be marinated ahead of time or simply laid of the grill, unseasoned, and then dressed after it is cooked. Grilled bok choy often serves as a vegetable to balance a meat-heavy meal, so substitutions should be aimed at filling this vegetable niche and also holding up well to a grill's high heat. Asparagus is an appealing substitute which is similar in color and similar in texture to a bok choy stem. Radicchio is similar in texture as well, although it is a chicory rather than a cabbage.
Bok choy in soups provides texture more than flavor. Unlike root vegetables, such as carrots, parsnips and onions, bok choy doesn't impart deep flavor to stock but rather is added late in the process, and it is lightly cooked to retain its color and texture. Spinach makes a suitable substitution, although it should be added in conjunction with a sturdier vegetable such as broccoli. Other types of cabbage, such as green, Napa or savoy, can also be substituted.