2) Preheat the oven to 200C/Gas 6.
3) Line the pastry with parchment paper and fill with pie weights or dried beans. Bake until the crust is just set, about 12 minutes. Remove the paper and weights and bake until the crust is light golden brown, 10 to 12 minutes. Remove from the oven and let cool on a wire rack.
4) Bring a large saucepan of salted water to a boil and add the asparagus. Cook until tender but not mushy, about 4 to 6 minutes. Transfer asparagus to a bowl of ice water to stop cooking. Drain and pat thoroughly dry. Cut asparagus into 2cm lengths. Discard any tough sections. Set aside.
5) Trim the root ends from the leeks and split in half lengthwise. Run each half under cool running water, making sure to remove all sand that is between the layers. Using the white and light green parts only, slice the leeks crosswise into 1/2cm slices. Save dark ends for another purpose.
6) In a medium saucepan melt the butter over medium-high heat, add the leeks and the garlic. Cook, stirring occasionally, until leeks are very tender but not browned, about 15 minutes.
7) In a mixing bowl, combine the double cream, eggs, salt, pepper and nutmeg and whisk thoroughly. Add the Parmesan and stir to combine.
8) Place half of the asparagus in the cooled tart shell. Top with the sauteed leeks, then add the remaining asparagus. Pour the egg-cream mixture over the top. Bake for 50 to 60 minutes, or until custard is set and tart is golden brown on top. If the tart starts to brown too much before the custard is set, lightly cover with a piece of aluminium foil until the tart is done. Remove the tart from the oven and evenly scatter pieces of the Vacherin over the top of the tart. Return the tart to the oven and cook for 2 to 3 minutes, or until the cheese is melted and gooey.
9) Remove from the oven and serve immediately, or serve slightly cooled.
For the basic savoury shortcrust:
1) Combine the flour and salt in a bowl. Add the lard and work it in with your hands until the mixture resembles coarse crumbs.
2) Add the water, 1 tbsp at a time, working it in with your hands. Add only as much as you need to make a smooth ball of dough. Wrap it in clingfilm and refrigerate for at least 30 minutes.
3) Remove the dough from the refrigerator and place it on a lightly floured surface. For 2 crusts, cut the dough in 2 and put the second half back in the refrigerator. For each crust, roll the dough out on the floured surface into a circle about 27cm in diameter and 1/4cm thick. Gently fold the circle of dough in half and then in half again so that you can lift it without tearing it, and unfold into a 23cm pie pan. Fill and proceed as directed in the recipe.