Prepare BBQ for cooking.

Heat the butter in a saucepan, add the leek and garlic. Gently sweat for 15 minutes, or until the leek is soft and without color. Season with salt and pepper, then set aside and allow to cool.

Lightly season the fish with salt and pepper. Cut out four squares of baking paper – you want the sheets to be around 30 cm × 42 cm wide. Preparing one sheet at a time, brush the centre of the paper with some olive oil and place a quarter of the leek mixture on top, followed by a fillet of fish.

Arrange the cucumber slices over the top of the fish fillet, positioning them closely together so they look like fish scales. Squeeze with a little lemon juice and add a pinch of dill.

Fold over one side of the paper to envelop the fish, making sure not to displace the cucumber slices. Using scissors, trim the two open edges into semicircles. Crimp the bottom and top edge together making firm creases to seal the parcel. Crimp most of the semi circle leaving the last 3cm unsealed the parcel should resemble a pasty; I like to blow air into the bag to puff it up. Pour in a little white wine, and then continue crimping the last corner to completely seal the parcel. Twist the ends to secure and place the parcel on a baking tray. Complete the remaining three parcels.

Place the tray on the BBQ and cook for 10 minutes. The bags should puff up if you have sealed them well. Remove the tray from the BBQ and allow the parcels to rest for 2–3 minutes.

Make the sauce by mixing the Japanese mayonnaise with the remaining chopped dill and lemon juice.

To serve, simply cut open the parcel and peel back the top, presenting the fish inside the parcel and topped with a dollop of the herb mayonnaise.