First make the filling. Heat the butter and oil in a heavy-bottomed pot over a medium heat. Add the onions and fry them for 15 minutes with the lid on. Add the salt, pepper and sugar and fry for a further 20 minutes, stirring occasionally, until the onions are a dark golden colour.
Add the flour, mixing it into the onions to ensure there are no lumps. Pour in the stock and vinegar, place the lid half over the pot and simmer for a further 15 minutes.
Preheat the oven to 220°C/200°C fan/gas 7.
While you wait for your onions to simmer, get on with rolling the pastry. This is a rare recipe where you don’t have to wait until all the ingredients are cold before you assemble your pie.
Cut your block of pastry in half and roll out 2 sheets of pastry to roughly the thickness of a pound coin (about 2mm). Layer the 2 sheets on top of one other. Turn the pudding bowl upside down on to the pastry and cut around the rim with a sharp knife. Remove the bowl and scraps and set aside one of the pastry circles.
Place a cheese slice in the centre of the other pastry circle and egg-wash the pastry around the cheese. Lay the other pastry circle over the top, so that the cheese is sandwiched between two circles of pastry. Prick a few holes in your cheesy pastry packet, and egg wash the top. This is your pie lid.
Bake your pie lid on a floured tray in the oven for 10 minutes. Remove when puffed up and golden.
When your onion soup is ready, remove it from the heat and stir in the Cognac. Ladle the soup into a warm bowl and pop the pastry lid on top like a giant cheesy crouton.
Hand out big spoons and devour immediately.