Make the pastry: put the flour in a large, wide bowl then add the butter, cut into small cubes. Rub the butter and flour together between your thumbs and fingertips until you have a breadcrumb-like texture. Alternatively, pulse for a few seconds in a food processor.

Add the parmesan, thyme and parsley then the egg yolks to the flour and butter. Combine, preferably with cool hands, adding enough water to produce a rollable dough. Wrap in kitchen film or greaseproof paper and chill for 20-30 minutes.

Make the filling: wipe and coarsely grate the courgettes, then transfer to a colander. Sprinkle them generously with salt, then place the colander over a bowl and leave for 30 minutes.

Trim the leeks and discard tough leaves. Cut them in half lengthways then slice into pieces the thickness of a pencil. Transfer to a colander and rinse under cold running water, then shake to remove any excess water.

Melt the butter in a large, wide pan over a low heat. Tip in the leeks, pour in the water then cover with greaseproof paper or baking parchment and a lid. Cook for 10-12 minutes, with the occasional stir. Stir in the peas.

Set the oven at 180C fan/gas mark 6.

Finely chop the chives and mint and add them, with the thyme, to the leeks. Stir in the lemon zest. Squeeze the courgettes firmly in your hands to remove as much water as you can, then stir into the leeks with a grinding of pepper and cook for four to five minutes. Break the mozzarella into large pieces, add to the filling and set aside.

Roll the pastry on a lightly floured board into a 40cm x 35cm rectangle, then transfer to a parchment covered baking sheet. Using a draining spoon, pile the filling into the centre of the pastry, leaving a 6cm rim of bare pastry around the edge and pressing the filling against the side of the pan to remove any liquid as you go. Bring the rim up and loosely fold over the filling, leaving an open area in the middle.

Brush the pastry with the beaten egg and the exposed vegetables with olive oil and bake for 35 minutes until the pastry is golden. Rest for 10 minutes before slicing.