The idea is that the apple makes it less mushroomy.  I like mushrooms, so used four big mushrooms but only two apples.   The stock was beef stock out of a packet (the liquid kind, as near to home-made as I can get).   The apples do add a certain tang to a rich soup, so equal amounts of mushroom and apple might have been better.  We decided that a little Worcestershire sauce at the end is just the ticket.