To make the pastry, place the flour and salt in a large bowl; add the butter and rub in using your fingertips until it resembles fine breadcrumbs.
Stir in the parmesan then the egg yolk (reserve the white to use later). Gradually add enough cold water to bring the mixture together into a stiff dough. Form it into a disc then wrap in clingfilm and refrigerate until firm, about an hour.
Roll the chilled dough out thinly on a lightly floured surface and use it to line a 20x30cm/12x8in rectangular tart tin. Press it right into the corners and up the sides; leave any excess pastry overhanging. Prick the base all over with a fork and place in the freezer for 15 minutes while you preheat the oven to 190°C/375°F/gas mark 5 and place a baking tray in the oven to heat up.
Line the chilled tart case with a sheet of baking parchment and fill with ceramic baking beans or dry rice. Make sure they go all the way into the corners and prop up the edges. Place the tart on the hot baking tray in the oven and bake for 20 minutes. Remove the baking beans and baking parchment and brush the base all over with the reserved egg white (this helps 'seal' the pastry and prevent the filling from making it go soggy).
Return the tart case to the oven for 10 minutes until the base is golden then remove from the oven, allow to cool for a couple of minutes then use a sharp serrated knife to carefully trim off the excess pastry from around the edge.
While the pastry is baking prepare the filling. Trim the ends off the asparagus so that they are the same length as the width of the tart tin. Bring a large pan of salted water to the boil and add the asparagus and peas, simmer for a couple of minutes until the asparagus is just tender then drain and rinse with cold water.
In a large jug, whisk together the eggs, cream and yogurt. Mix in the crushed garlic, grated parmesan, chopped mint, thyme, sliced spring onions and some salt and pepper then add the feta and peas.
Turn the oven temperature down to 180°C/350°F/gas mark 4. Pour the filling into the tart case and make sure it is level. Arrange the asparagus spears down the middle of the tart, alternating which direction the tip faces. Bake the tart for about 40 minutes, until it is set in the middle. Remove from the oven and allow to cool for 20 minutes before serving.
