Method

1. Preheat the oven to 180°c (fan-assisted).

2. Melt the butter in a large heavy-bottomed pan over a medium heat. Add the sliced leeks and a good pinch of salt and cook, stirring regularly, for 12-15 minutes, or until they have collapsed and are starting to brown. Finish with a few good grinds of black pepper, then taste and adjust the seasoning, if needed.

3. Meanwhile, lay the pastry, still on its baking parchment, onto a baking tray. Use a knife to indent a 1/2 inch border all the way round the pastry, then brush with some of the leftover egg whites and bake for 10 minutes.

4. Combine the egg yolks, crème fraîche, mustard and a good pinch of salt and black pepper in a bowl. Set to one side until you are ready to build the tart.

5. Sprinkle the grated Gruyère evenly over the base of the par-cooked pastry, leaving the border clear. Top with the leeks, then pour the crème fraîche mixture over them. The mixture is quite fluid, so aim for the centre; the leeks will help keep it from running over the sides - though, if it does, don't worry. It may look a little rustic, but it will taste great. Bake for a further 18-22 minutes, or until the pastry is puffed, golden and cooked through. Let it cool for 5 minutes.

6. Grate a lot of Parmesan over the top, more than you think. It should be covered with Parmesan! Finish with a crack of black pepper, then slice into squares, and eat warm.