Uuz is Mutton Back that is stewed in one piece. A well fed sheep will collect substantial fat in the lower back and in the tail as an energy reserve. This fat is the main component of Uuz.
Large filled pockets that are deep fried in mutton fat or Khuushuur.
Buuz are small filled pockets, cooked under steam, and usually have an opening at the top.
Daniel got quite sick eating Byyz, which sounds like it is Buuz, but who knows?