Heat the butter in a large frypan over med-high heat.  Cook the mushrooms until soft, then set aside.

Leave the frypan on the heat, place into it one tortilla, layer it with half the grated cheese, mushrooms, feta, top with remaining grated cheese and then finally the other tortilla. 

Cook for 2-3 minutes on each side until lightly browned and the cheese has melted.

Remove from pan and cut into 4 triangles.