Whole Sheep Roast is a famous dish from the Tang era of the Celestial Empire, recorded in the Taiping Guangji. To prepare this dish, one must stuff a slaughtered goose with meat and rice and then place it inside a likewise slaughtered sheep's stomach to roast together. When eaten, the sheep was discarded, and only the goose was consumed.
In the Tang Dynasty, this was an extremely popular dish, said to have originated in the royal courts and even featured at the banquets of the Tang Emperor.
As for why this dish would appear at a Japanese deity's feast, Chen Yu speculated that the envoys sent to the Tang Capital Chang'an must have encountered the dish and brought the recipe back to Japan.