Soak mushrooms in warm water for about an hour. Drain and saute in 1 tblspn of the oil over a low heat for 5 minutes. Saute the onions in 2 tblspns of the oil until tender. Add salt and pepper. Preheat oven to 180c. Layer the potatoes, onions and mushrooms alternately in a large greased baking dish. Sprinkle each layer with parmesan, salt and pepper. Pour the milk over the potatoe mix and bake for about an hour, or until the liquid is absorbed and the crust is lightly golden. Rest for 5 minutes before serving.
