Soak the brown rice in salted water overnight, strain and rinse. Put it in fresh salted water and cook gently until tender, 15-20 minutes. The grains should still be separate and have a slight bite to them.
Meanwhile slice the mushrooms and put them in a sauté pan with the butter. Let them cook very, very slowly so that the juices all run out; stir from time to time. Chop the parsley an