DIRECTIONS On a lightly floured work surface, roll out the pastry to a circle just under ¼-inch thick. Line an ungreased fluted removable-bottom tart pan, pressing into the edges well and trimming away any excess dough. Chill the pastry for 30 minutes.
Preheat the oven to 400 °F (200 °C). Place the chilled pastry shell on a parchment-lined baking tray. Pierce the bottom of the crust with a fork to prevent air pockets from forming. Place a piece of foil completely over the crust and fill the shell with raw rice or beans (or pie weights).
Bake the crust for 10 minutes, then remove the foil with the weights and bake 10 minutes more. The crust will only brown at the edges, less than a traditional pie crust. Allow the crust to cool while preparing the filling.
. Reduce the oven temperature to 350 °F (180 °C). Melt the butter in a sauté pan over medium heat and add the leeks, sautéing until tender, about 10 minutes. Sprinkle the leeks over the bottom of the cooled crust and top this with the grated Gruyere.
Whisk the eggs lightly, then whisk in the nutmeg, salt and pepper (most of the seasoning comes from the salty cheese). Pour this slowly into the crust, allowing the filling to seep in between the onion and cheese sprinklings. Bake the quiche for 40 to 45 minutes, until it no longer jiggles when gently moved. Allow the quiche to cool for 15 minutes before slicing to serve.
Alternatively, the quiche can be baked a day ahead and chilled, then warmed in a 325 °F (160 °C) oven for 15-20 minutes before serving.