Adjust oven shelf to middle position and preheat to 190°C.
Fill and switch on your electric kettle.
Oil a medium size baking dish.
Place fish portions in the dish.
Season very lightly with Fish Spice.
Decant baby spinach into a colander.
Pour a whole kettle of boiling water over spinach to wilt the leaves.
Squeeze the leaves still, in the colander, by placing a saucer, right side up, on top and press down hard to squeeze liquid out of spinach.
Turn spinach out onto a wooden board and roughly chop with a mezzaluna or sharp knife.
Season very lightly with Vegetable Spice and spread spinach over the fish.
Briefly sauté the mushrooms in the oil and butter.
Stir in the Cheese Sauce and spread over the spinach layer.
Sprinkle with grated cheese.
Bake for 20 minutes.
Serve with crushed baby new potatoes and a salad.