To make the pastry: sift together the flour, salt and mustard powder. Cut butter into small cubes and add to the flour. Rub lightly between your fingertips until well combined. Gradually add the water until a soft dough is formed. Turn out the pastry onto a board and shape into a small ball. Put the dough back in the bowl and cover with a cling film, place in the refrigerator for at least 30 minutes. Roll out on a board dusted with flour, to line a 230mm diameter pie dish. Refrigerate for a further 30 minutes and bake blind, by covering the pastry with greaseproof paper and covering with beans or rice. Bake in a preheated oven at 200 celsius for 9 minutes then remove paper and rice and bake for a further 5 minutes.

To make the filling: Saute the leeks and heated butter and cooking oil until transparent. Scatter the cooked leeks on the base of the pastry case. Mix remaining ingredients in a bowl and when combined pour into the pastry case. Bake at 160 celsius for 35-45 minutes until filling is well set.