Preheat oven to 180°C.
For the sauce,heat butter and olive oil in a saucepan over medium heat. Add the leek,fennel,garlic,a pinch of salt and ground white pepper and the cayenne. Cook over a low heat until the vegetables are soft. Stir in the flour and cook until it bubbles and goes grainy.
Gradually add the wine,stirring constantly,then gradually stir in the milk. Cook,stirring over a low heat,until the mixture thickens. Stir in the mustard,cream,lemon zest,dill and parsley and simmer for 2 minutes. Check the seasoning and adjust as necessary.
For the pies,steam the fish in a bamboo steamer for about 3 minutes.
Place a ramekin on top of a puff pastry sheet and,using the ramekin as a guide,cut around it to create a circle,cutting 1cm larger than the dish on all sides. Repeat 3 times and place the 4 pastry rounds in the fridge for 20 minutes to allow the pastry to rest.
Brush the pastry rounds with egg wash,place on a baking sheet and bake at 180°C for 20-25 minutes.
Place 2 tbsp of sauce into each of the 4 ramekins,then distribute the steamed fish carefully on top. Cover with remaining sauce. Put the ramekins in the oven at 180°C for around 6 minutes. Time it so the puff pastry lids finish at the same time as the pie fillings. Place the pastry lids on top of the pies and serve.
Photography by William Meppem. Food styling by Hannah Meppem. Food preparation by Nick Banbury.