If you've been to a real Luau you know about the Imu and one of my characters describes the process in Penniless Hearts. It's usually used to roast a pig but they also use it for fish and vegetables. Typically they use taro leaves to wrap the food before it's put into the imu. Taro leaves are literally called Luau but the word luau has become known as a Hawaiian feast.
Basically, the definition of Imu is an underground pit used for cooking.