Preheat the oven to 170ºC
Layout the fish in a line to help you judge how much tin foil and parchment you need.
You will need some tin foil about three times bigger than the fish and a sheet of parchment that is a just a bit smaller than the foil.
Place the foil on a flat surface, place the sheet of parchment on top of the foil. Spread the leeks along the center of the parchment (make sure to leave about an inch free at each side). Place the chorizo slices on top of the leeks. Drizzle this with some oil.
Season the fish with salt and pepper and then place the fish on top of the chorizo. Add the zest and lemon juice to the top of the fish. Carefully fold the parchment over the fish, roll the edges of the parchment to seal. Fold the foil over the parchment envelope and roll the edges to seal it.
Place this on a tray and put in the oven.
It should take around 10-12 minutes for the fish to cook depending on the thickness.
Remove from the oven and carefully open the foil and parchment.
Use a spatula to divide the leeks, chorizo and fish between four plates.