1. Wash and slice the leeks into rounds, slice the onion. In a large skillet, sauté the onion, leeks, and bacon in a little butter until they change color.
2. In a bowl, beat the egg with 20 cl of heavy cream. Add the leek mixture to the bowl and season with pepper.
3. Roll out the puff pastry in a tart dish and prick it. Pour the contents of the bowl onto the pastry.
4. Bake in a preheated oven at 180°C (350°F) for 25 minutes.