Chicken is marinated in a collection of herbs and spices then grilled over an open flame. It's served over a bed of broken rice (like normal rice but broken up into smaller pieces) and often accompanied by some vegetables -- tomato and cucumber slices, pickled daikon and carrot, etc. Just watch for a busy place with a man standing in front of a grill. It shouldn't be hard to find - cơm tấm gà nướng is a popular lunch time meal.