Steamed Fish and Mushroom in a bag （お魚ときのこのフォイル焼き）

This is one of the most common way to cook fish in Japan, and it's the easiest one.
I recently remember this method from a book called "Lunch in Paris". The author is American lady and living in France. I don't know where she came to know this recipe, but I've never seen it anywhere else other than Japan or China.
How to make is very easy, but this is an experience: As soon as you open the bag, all the nice flavor burst into the air.

How to make :
1: place white fish, mushroom, and onion slice in a alumni foil.
2: Close the foil to make a bag.
3: Place it in toaster oven, and bake it for 30 min.
4: Squeeze lemon and put soy sauce on it before you eat.