Sauté the onion and garlic in the oil, add the mushrooms, leave uncovered and cook together for approx. 10 mins. until the resulting liquid has completely boiled off. Mix in the lemon zest, lemon juice and parsley, season, and leave to cool.
Fish & potatoes
Rinse the fish (inside and out) in cold water, pat dry, season inside and out. Stuff with approx. half of the mushrooms, put the rest of the stuffing to one side.
Wrapping: Put half of the potatoes and half of the remaining stuffing in the middle of each sheet of baking paper. Place the fish on top, pour over the wine, and place knobs of butter on top. Bring the front and rear edges of the paper together above the fish, make an approx. 1 cm fold, then fold over once more, tie the ends with cooking string (not too close to the fish), place on a baking tray.
Oven bake for approx. 30 mins. in the centre of an oven preheated to 180°C, then remove.
Serve: place the parcels on a plate, open the paper, garnish the fish with lemon.