Preheat the oven to 200C/400F/Gas 6.
Cut a 30cm/12in circle out of baking paper and another the same of tin foil. Lay the baking paper circle on top of the foil and fold in half (so that the baking paper is in the middle).
Open the paper back up and lay the lemon slices in the middle of one half.
Place the halibut on top of the lemon slices and season with salt and freshly ground black pepper.
Place the spinach on top, and pour over the wine.
Fold the clear half of paper and foil back over the ingredients and seal the edges by scrunching the foil tight.
Place onto a baking sheet and cook for 12-14 minutes in the oven, until the halibut is cooked through.
To serve, place the parcel onto a plate and slash the top open with a knife to let the aroma escape.