Cut the pre-cooked sausage into small pieces and do the same with the onion, garlic, and leek.
Heat up the vegetable oil in your Aga stockpot and add the above ingredient, the mustard and then some salt and black pepper.
Add the beef stock, bring to the boil then place the stockpot in the Simmering Oven for approximately 3 hours.
Line the pie dish with the rolled out puff pastry before putting in the filling.
Then cover with a sheet of pastry, making a vent in the top of the pie and brush with a beaten egg.
Cook on the grid shelf on the floor of the Roasting Oven for about 25 minutes until its risen and turned golden.