Step 1

For the pastry, place flour and a pinch of salt in a bowl. Using your fingers, rub in butter until it resembles coarse crumbs. Add egg yolk and thyme. Combine vinegar and 1/4 cup (60ml) water in a jug with 4 ice cubes. Drain, then fold into flour mixture until it forms a shaggy dough. Enclose in plastic wrap and chill for 2 hours.

Step 2

Preheat oven to 200°C.

Step 3

Roll out pastry on a lightly floured work surface to 3mm thick and use to line a 24cm fluted ceramic dish, trimming excess. Line with baking paper and fill with pastry weights. Bake for 25 minutes or until light golden. Remove weights and paper, and bake for a further 20 minutes or until base is cooked.

Step 4

Heat oil in a frypan over medium-low heat. Add leek and prosciutto, and cook, stirring, for 10 minutes or until softened. Transfer to a bowl and set aside to cool.

Step 5

Whisk egg, milk and cheeses in a bowl until smooth. Season. Stir in leek mixture, pour into pastry case and scatter with asparagus. Bake for 1 hour or until just set.

Step 6

Cool slightly, then scatter with onion, parsley and baby parsley and mint to serve.