Preheat the oven to 180 C degrees or heat your BBQ.
Cut out a large piece of foil - big enough easily to wrap the fish.
Cut a slightly smaller piece of baking paper and put this on top of the foil.
Cut the leek an lemon into 1 cm thick rings and layer them in two rows in the middle of the baking sheet. Place the fish on top.Season with salt and pepper. Drizzle with the oil, or crumble the butter all over the fish.Scatter the herbs on top.
Wrap the fish, making a tight parcel - no liquid should be able to escape and the foil shouldn't touch the fish.
Put the parcel on a baking tray and bake for 20 minutes. Alternatively put the parcel on the BBQ ( not on direct flames), and cook over low heat for approx. 25 minutes.
Carefully unwrap the fish and serve warm with the leek and cooking jus.