Heat the oven to 200°C/Gas 6. Heat the oil or butter in a heavy-bottomed saucepan and add the leeks and thyme. Slow-fry the leeks until they are very soft and starting to brown, a good 10–15 minutes.
Cover the pan initially to help them sweat, then take off the lid halfway through so the liquid evaporates. Stir at intervals to stop them catching. Season with salt and pepper then cool.
Meanwhile, lay out your pastry flat on a lightly greased non-stick baking sheet and bake for about 10 minutes until it has completely puffed up and is golden brown. (Check the bottom of the pastry is cooked too.)
Flatten the pastry back down by covering it evenly with the leek mixture, leaving 5mm around the edge. Sprinkle with the Parmesan and any other toppings (see suggestions below) and return to the oven for a further 5 minutes, until the cheese has melted. Serve warm.
* Onions work as a replacement for or combined with the leeks.
* Experiment with extra toppings, just like a pizza: try anchovies, olives or different cheeses, such as mozzarella or goat’s cheese.