Preheat the oven to 200°C/gas 6. Sift the flour with a pinch of salt into a large bowl. Add 5 tbsp cold water to the oil, then stir into the flour using a fork to mix together and form a dough. Sprinkle with a little more water if necessary. Knead the dough briefly on a lightly floured work surface.
Roll out the dough to about 3mm thickness. Using a round metal cutter (or cutting around a saucer), cut out 4 circles, each about 15cm in diameter. Re-roll the trimmings as necessary. Carefully fit the pastry circles into 4 individual, 12cm diameter tartlet cases.
Put the bacon and oil into a non-stick frying pan and set over a moderate heat. Cook for 2–3 minutes, then add the sliced leeks and rosemary. Reduce the heat and cook gently for about 5 minutes, stirring frequently, until the leeks are tender. Remove from the heat and leave the mixture to cool in a bowl while making the sauce.
Mix 2 tbsp of the milk with the cornflour in a heatproof bowl. Heat the rest of the milk in a medium pan. As soon as it comes to the boil, pour it onto the cornflour mixture, whisking constantly. Tip this mixture into the pan, return to the heat and stir constantly until the sauce boils and thickens. Add the sauce to the leek and bacon mixture and season to taste (you may not need to add salt as the bacon is salty).
Whisk the egg whites in a clean bowl until they stand in soft peaks. Using a large metal spoon, gently fold the egg whites into the leek and bacon mixture. Carefully spoon into the pastry cases.
Bake for about 20 minutes until the filling is golden and set and the pastry is crisp. Serve immediately, garnished with little sprigs of rosemary.