For this burger, which has become what we call a "pub favorite" in America, we again combine ground chuck with ground sirloin. If you prefer, you can use one or the other, but remember that sirloin alone will be a little drier than the combination. However, the bacon, although cooked, and the mushrooms add a little moisture. The chuck alone will produce a juicy and plump burger but will lack a little of the deep flavor of the sirloin.