Method

1. Preheat oven to 180ºC. Melt butter in a large, heavy-based pan over a medium heat. Add leek and cook, stirring regularly, for 5 minutes. Stir in flour and cook, stirring regularly, for 2 minutes, then pour in wine and stock and bring to a simmer. Cook for 3 minutes, then fold in celeriac, dill and cheese. Transfer to a large tray and put in the fridge for 1 hour.

2. Mix in fish pieces and cream, then season with salt and pepper. Spoon into a deep, 20cm-diameter (base) baking dish. Brush both sides of pastry sheet with egg and place over fish mixture, pressing down and sealing around edge. Sprinkle with sesame seeds, cut a small hole in the centre to release steam as the pie cooks, then bake for 30 minutes or until pastry is crisp and golden. Allow to cool for 2 minutes before serving.