To make the pastry, place the flour and salt into a mixing bowl, then add the butter
Using your fingers, rub the butter into the flour until it resembles breadcrumbs
Add the egg and mix until a dough is formed. If it is too dry, add a splash of milk. Wrap the dough in cling film and chill in the fridge for 30 minutes
Meanwhile, pre-heat your oven to 180C Fan/Gas Mark 6. Cook the leeks and onion in a pan on a medium heat along with the vegetable oil and nutmeg. Once the onions and leeks are soft, take them off the heat and set aside to cool
Roll out the pastry on a floured surface. Place it into an 9" loose bottomed tart tin and trim the edges, then prick the bottom all over with a fork
Cover the pastry with baking paper (scrunch the paper up first to make it easier) then add baking beans (or dried rice or beans). Bake the pastry for 15 minutes, then remove the baking paper and beans, and bake for a further 10 minutes
In a food processor blitz the cooked onions and leeks along with the single cream, eggs, pecorino, sage and salt
Pour the filling into the pastry case, then arrange the brie slices on top
Sprinkle some chopped up fresh sage on top, then bake the quiche for 35-40 minutes or until golden brown
Serve warm or allow to cool and store in the fridge for 3-4 days