1. Place the cod, milk and bay leaf in a large saucepan and bring to a simmer over a medium heat. Simmer for 4-5 mins until the cod is just cooked. Strain, discarding the bay leaf. Transfer the cod to a large mixing bowl and flake. Cover and set aside.
2. Melt the 25g butter in a large, heavy-based saucepan over a medium heat and sweat the onion, leek and courgette for 8-10 minutes, stirring. Stir in the parsley, then the flaked cod and remove from the heat. Seasoning to taste.
3. Preheat the oven to gas 6, 200°C, fan 180°C. Roll half the pastry out on a lightly floured surface into a large square about 0.5cm thick. Transfer to a large lined baking tray. Spoon the cod and vegetable filling into the centre, leaving a 3-5cm border all the way around.
4. Roll the other half of pastry out into and identical square with ½cm thickness. Brush the edges of the base layer of pastry with some of the beaten egg. Carefully sit the other square of pastry on top and seal at the edges. Lightly score zig-zag patterns all over the top of the pastry with a knife, then brush with the remaining beaten egg. Bake for 20-25 mins until golden and risen.
5. Meanwhile, make the chive sauce by warming the crème fraîche gently in a small saucepan over a low heat until smooth and runny. Stir in the chives and butter and whisk until smooth. Add the lemon juice and some seasoning to taste. Set to aside.