The first is to prop the pork on a roasting rack for even heat distribution. If it’s not elevated, it will get wet and soupy on the bottom instead of burnished and sticky.
The next thing to do is to line the rimmed sheet with foil and fill it with a cup of water. The water will maintain a moist environment and the sheet will catch any drippings.
To get the charred finish, roast the char siu between 425 and 475 degrees F on an upper rack. The edges should start to singe and change color without actually burning.