I know of a fast food joint that sells basicallly one thing -- mushroom burger. It became popular with city folk, and the outlet is still alive up to now. You can also buy fresh mushrooms from them.
I don't have a recipe for their burger but I should think that from the taste, you can just replace half or more of the ground beef in your usual recipe with ground/finely chopped mushrooms. The patty comes out very moist and tasty.
'Never eat more than you can lift.' - Miss Piggy