1 Heat the oven to 200°C (180°C in a fan oven), 400°F, gas 6.
2 Season the fish with salt and ground black pepper. Mix together the mustard, lemon juice, 2 tbsp oil and some salt and ground black pepper.
3 Place two sheets of parchment paper on top of each other and spread with oil. Place the leek and the lemon slices on top and season with salt and ground black pepper. Place the fish on the paper and drizzle with the mustard-oil mixture. Arrange the olives, thyme and garlic on top.
4 Fold the paper over the fish and close well. Place on a baking tray and bake for around 25 min. Remove from the oven and serve.