Stargazy pie white fish will work for the filling, with pilchards or herring just added for the presentation. Prior to putting it in the pie the fish should be skinned and boned (except the head and tail) to make it easier to eat. Along with the fish, the other traditional ingredients are thickened milk, eggs and boiled potatoes. There are many recipe variations around the traditional ingredients, some of which include hard-boiled eggs, bacon, onion, mustard or white wine. Other alternatives to the main fish can be crayfish and rabbit or mutton. The stargazy pie is always topped with a pastry lid,
Pie and mash eels were one of the few forms of fish that could survive in the heavily polluted River Thames and London's other rivers at that time. Supply was plentiful through the late 1800s, particularly from the Dutch fishing boats landing catches at Billingsgate Fish Market. Since 2010, as revealed in a joint study by the Zoological Society of London and the Environment Agency, the number of eels captured in research traps in the River Thames fell from 1,500 in 2005 to 50 in 2010, meaning most eels used in pie and mash shops are now from the Netherlands and Northern Ireland.