1 First make the pastry. Place the butter and flour into a food processor and blitz until it resembles fine breadcrumbs. You can also do this using your fingertips or a round-bladed knife. Add the egg to combine until a dough forms.

2 Place the dough onto a cold work surface, pat gently into a rounded shape, cover with cling film and leave to rest in the fridge for 30 minutes.

3 Preheat the oven to 200 degrees (180 fan), or gas mark 6.

4 Trim the rough green part from the top of the leek, then halve and rinse the leeks under the kitchen tap to remove any grit. Thinly slice the leeks.

5 Place the butter in a medium, heavy-based pan and add the leeks. Leave to sauté for 10-12 minutes, stirring frequently, until the leeks are tender and have melted down.

6 Add the crème fraiche or cream. Simmer until the sauce is thick, then stir in the cheese. Season with salt and plenty of black pepper. Remove from the heat and leave to cool.

7 Meanwhile roll out the pastry on a lightly floured worktop. Roll roughly into a 16-inch circle or rectangle. Place on a lined baking tray.

8 Once the leek mixture is cooled slightly, stir through all but one tablespoon of the beaten egg, mustard, thyme and chopped parsley.

9 Spread the leek mixture on top of the rolled out pastry disc. Leave a two-inch border around the edges. Pleat the pastry edges up over the filling so it forms a crust and holds in all the filling. Sprinkle the za'atar over the filling. Brush the edges of the pastry with the remaining beaten egg and bake for 25-30 minutes, until the pastry is golden and the filling gently set. Garnish with fresh thyme sprigs.