For the pastry, combine the flour, xanthan gum, salt in a food processor.
Add the butter and pulse to rub the butter into the flour but allow some small streaks of butter to remain. Tip this into a bowl and add the egg and water then mix and gently knead to smooth pastry. Wrap and transfer to the fridge for at least an hour to rest and chill.
Meanwhile, heat the olive oil and butter in a medium pot and add the onion, carrot, potato, garlic and a good pinch of salt. Sweat the veggies off for about 5 minutes then add the flour and stir through, cooking for another minute. Heat the stock in a small saucepan.
Add the wine to the veggies, stir through and then add the hot stock, little by little, stirring constantly to a achieve a smooth, thickened sauce. Cook over a medium heat, stirring often, until the potato is soft then add the cream and cook for 2 more minutes. You should have a thick sauce. Stir through the mustard and peas. Transfer the mix to a bowl and allow to cool. Once cool fold through the fish, herbs and season well with salt and pepper. Set aside in the fridge.
Cut the dough into 8 even pieces, roll 4 of them between baking paper into circles big enough to line the pastry tins. Press this into the tins them spoon in the fish filling to the top, no higher.
Roll the remaining dough to be big enough to cover the top of the pies and place over the filling. Press the edges together to seal and use a small sharp knife to trim the edges and neaten. Use a fork to press around the pie edge and then use any remaining pastry to decorate the top with shapes or letters.
Brush the pies with the egg yolk and bake in a 190C preheated oven for 25 minutes.
Serve straight away or cool, refrigerate and reheat thoroughly when needed, approx.. 10 minutes in a 190C preheated oven