For the pastry, preheat the oven to 200C/400F/Gas 6 and line a large baking tray with greaseproof paper. Roll out the pastry on a lightly floured surface and cut into 30 x 6cm/2½in squares. Score a smaller square inside each one, leaving a 75mm/¼in border. (Take care not to cut the pastry all the way through.)
Bake the pastry squares in the oven for 18-20 minutes, or until puffed up and golden-brown.
Remove from the oven and transfer to a wire rack to cool. When cool carefully remove the central square of pastry from each one.
Meanwhile, for the smoked haddock and prawn filling, place the haddock in a saucepan with the milk, bring to a simmer, switch off the heat and leave the fish to cook in the residual heat for ten minutes.
Drain the haddock, reserving the milk. Set the haddock aside to cool slighly, then remove the skin.
For the sauce, pour the reserved milk into a saucepan and add the onion, bay leaves and thyme. Bring to a gentle simmer, then remove from the heat and leave to infuse for ten minutes. Strain the milk through a sieve into a jug and discard the onion and herbs.
Melt the butter in a non-stick saucepan over a low heat, add the flour and cook for a few seconds, stirring constantly. Stir in the hot infused milk slowly, and cook for 1-2 minutes or until the sauce is smooth and thick. Season, to taste, with salt and freshly ground black pepper, remove from the heat and stir in the crème fraîche. Cover with clingfilm and leave to cool.
Divide the sauce into two portions, flake the haddock into one half of the sauce, checking for any remaining bones and stir in the prawns and chives. For the chicken and tarragon filling, stir the chicken, tarragon and wine into the other half of the sauce.
To assemble the vol-au-vents preheat the oven to 200C/400F/Gas 6. Put the pastry squares onto a baking tray, spoon in the fillings and warm in the oven for 8-10 minutes or until the filling is bubbling. Serve warm or cold.