Dating from the times when the Thames was cleaner and full of life, these pies would originally have contained eels - the accompanying sauce is made from the liquid (the liquor) the eels were cooked in, along with a load of parsley that gives it its green colour. Nowadays the most common filling is minced beef, although finely diced beef can also be used.
Remove from heat and allow to cool completely - you don't want to be adding hot filling to pastry, it'll create sogginess in the bottom area.
Otherwise, dissolve a fish stock cube in hot water, bring to the boil and reduce to a simmer. Add enough water to the flour and stir well to make a smooth slurry. Add this to the stock (if using eels to make your stock, remove them and set aside), stir well and keep on stirring until the sauce has thickened.
Add a dash of chilli vinegar and taste for seasoning. Add salt if needed.
On a floured worktop, roll out shortcrust pastry for your lids. It should be about 3-4mm (just under a quarter of an inch) thick.If you have a cutter or bowl that is the same outside diameter as your dishes, use that to cut lids. Otherwise cut around an upturned pie dish with a knife. Use the narrow end of a funnel to cut a hole in the centre of each lid.
Roll out suet pastry for the bases. Gently press the pastry into each dish and trim off any excess pastry.
Fill each pie casing with the cooled minced beef filling. Moisten the rim of the base with cold water and put a lid on. Press it down all around to form a good seal, then lift up the edges of the pies so that the pastry doesn't stick to the tins.
Brush each pie with beaten egg to glaze it.
Place your pies into a large roasting tin and add enough very hot water to come at least halfway up the pie tins. Tightly cover the roasting tin with aluminium foil and cook for 30 minutes. Remove the foil and continue cooking for about another 15 minutes. The idea of this last bit of cooking is to brown the tops of the pies.