For the pastry, place the flour and butter in a food processor and blend. Add the egg yolk and 3-4 tbsp chilled water, blend until you have a smooth dough. On a floured surface knead briefly, cover in cling film and chill for 30 minutes.
Preheat the oven to 190°C/170°C Fan/Gas Mark 5.
On a lightly floured surface, roll out the pastry and line a 23cm deep flan tin, press the dough into the sides. Prick the base of the pastry and line with greaseproof paper and baking beans. Blind bake for 12 minutes, remove the paper and beans, and bake for a further 10 minutes or until slightly golden.
For the filling, heat a large frying pan and dry fry the lardons for 3-4 minutes. Add the onion, fry for a further 2-3 minutes then spread over the pastry case and top with the cheese.
In a bowl whisk together the eggs, double cream, milk and pepper, and pour into the pastry case. Place in the oven and bake for 25-30 minutes, or until golden and set.
Allow to cool slightly and serve while still hot or cold.