Caramelized onions get a flavor enhancement from fresh rosemary.
In a bowl, toss onions, rosemary, olive oil, and balsamic vinegar; season with salt and pepper.
Transfer onions to hot wok, reserving bowl and marinade; cook onions, tossing frequently, until golden and caramelized, 20 to 40 minutes.
Return onions to bowl; toss with reserved marinade.
The sloped sides and perforated surface of a grill wok allow you to grill small foods, such as shrimp and vegetables, that might otherwise slip through the grates.