Peel the prawns and set aside; reserve the heads and shells. Heat a large, deep pan and add 2 tablespoons of the vegetable oil. Dust the rabbit legs with salt and half the flour, then add to the pan to brown all over. Once browned, remove and set aside.
Add the remaining oil to the pan, then sweat the onion until soft, without colouring. Add the prawn heads and shells, garlic, fennel seeds, peppercorns, star anise, bay leaves, thyme and bunch of tarragon sprigs. Cook for 5 minutes. Stir in the tomato puree and wine and cook for a further 5 minutes. Add half of the chicken stock and bring to the boil, then add the remaining stock and bring up to the boil again. Return the rabbit legs to the pan. Simmer for 1 hour, until the legs are cooked through.
Meanwhile, make the pastry. Combine the flour, butter and salt in a food processor and blitz until the mix resembles breadcrumbs. Add just enough iced water to bind, blitzing briefing to combine. Remove from processor and shape into ball. Wrap and leave to rest in the fridge for half an hour before rolling out.
Remove the rabbit legs from the cooking liquid and allow to cool slightly, then remove the meat from the bone, keeping it in good-size chunks. Set aside. Strain the cooking liquid through a fine sieve into a bowl, pressing on all the flavourings to get the most out of them.
Melt the butter in a saucepan, add the remaining 50g flour and mix well. Add a little of the strained cooking liquid and whisk well, then add the remaining liquid. Bring to the boil and simmer gently for 5 minutes. Taste and adjust the seasoning, then finish with the chopped parsley, tarragon and mustards. Add the prawns and rabbit chunks and mix into the sauce. Set aside.
Preheat the oven to 180C/350F/gas mark 4.
Roll out the pastry on a lightly floured surface to 4mm thick and cut to the required shape to make a lid for your pie dish (or dishes) – for a large pie, use a 22cm diameter dish that is 4cm deep, or for individual pies, use four 11x5cm oval dishes that are 3.5cm deep. Spoon the filling into the pie dish(es). Brush the rim of the pie dish with egg yolk, then bake for about 20 minutes, until golden brown. Allow to rest for 5 minutes before serving.