Mix the ginger, tamari and lemon juice together in a small bowl. Cut large pieces of baking foil and fold them into two foil parcels. Place the dory into each parcel and pour the marinade into each parcel.
Wrap the fish up loosely in the foil. Place in a steamer for 15 - 20 minutes. Prepare the vegetables by washing and trimming the spinach and brushing and slicing the mushrooms. Sauté the mushrooms in olive oil for 10 minutes, then drop the spinach into the pan and stir through until it's wilted and cooked through. Place the fish and sauce on top of the vegetables and serve immediately.
Note: When buying mushrooms always use the paper bags provided and refrigerate them in the bags away from foods with a strong odour. You can peel mushrooms to clean them but as the skin offers additional nutrients it's better to wipe them clean with a damp paper towel. Never wash them as they are so porous they'll absorb the water becoming slimy.