Sift the flour into a large bowl and grate in the butter. Using your fingertips, rub the butter into the flour, lifting as you go to aerate. Leave some lumps of butter to make the pastry flaky. Using a fork, quickly stir in enough iced water to form a soft dough. Pat out dough to a 15cm diameter on a sheet of plastic wrap and refrigerate for about 30 minutes. Roll out dough and line a 23cm tin or quiche dish. Trim the edges and reserve the offcuts (in case you need to do some patching later). While dough is resting, preheat the oven to 190°C and prepare filling. Place the pancetta or bacon in a large heated frying pan (a little extra pancetta or bacon is always nice, so you can be generous here). Let the fat render a little, then add the leeks (add a teaspoon of oil if necessary) and sauté until translucent. Place the eggs and extra yolk in a bowl and beat well with a fork, then stir in the cream, cheese and nutmeg and season well with salt and pepper - a tablespoon of Simon Gault Home Cuisine Italian seasoning here will add 5 percent extra magic. Place a sheet of baking paper on the pastry case and fill with ceramic beads or dried beans to weigh it down. Bake for about 15 minutes, then remove paper and weights and bake for a further 5–8 minutes until lightly golden. Reduce the oven temperature to 170°C. Combine the pancetta and leeks with the egg mixture and pour into the pastry case, then bake for 30–35 minutes until puffed, golden and set. Cool for a few minutes before cutting into wedges to serve.