1.FOR THE DOUGH: Process flour and butter in a food processor and pulse until butter and flour form small pearl size granules about ten seconds of pulsing. Or do this by hand using a fork or a pastry tool. Mix flour and butter until breadcrumbs form. Don't over handle butter and flour, mix slowly and gently.
2. Add ¼ cup ice water until combined. Pulse 1-2 seconds or mix by hand using your fork or pastry tool. Pinch dough with fingers; if dough is floury, dry, and does not hold together, add 1 to 2 tablespoons ice water and mix until dough comes together in a loose ball.
3. Turn dough out onto clean and floured work surface and flatten into 5-6-inch disk; don't work the dough. The less it is handled the flakier the pastry. Cover disk in plastic wrap and refrigerate 1 hour, before rolling. (Dough can be refrigerated for up to 24 hours but then you need to thaw is 10-12 minutes before rolling.

4. Roll out disk of dough on floured work surface to 12 by 8-inch rectangle or 11 inch round (about ¼ inch thick). Roll dough loosely around rolling pin and unroll into quiche pan. Working around circumference, ease dough into pan. Trim any dough that extends more than 1 inch over edge of pan. Patch any cracks or holes with leftover.
Press dough gently to fit into pan. Using a fork press holes all over the dough. If you have pie weights you can use these instead just before the crust goes into the oven.
Refrigerate dough-lined pan until firm, about 20 minutes, and then freeze for 30 minutes.
6. Adjust oven rack to middle position and heat oven to 350 degrees. Fill with pie weights or dried beans.
Use strips of foil or a pie protector to cover the edges of the pastry. This prevents excessive browning.
Bake until exposed edges of dough are beginning to turn golden 35- 40 minutes. Carefully remove quiche pan and empty out pie weights.
Remove shell from oven. Lower temperature to 325.
Brush hot interiors of pastry lightly with a little whisked egg using a pastry brush. Cool shell 35 minutes.
7. FOR THE FILLING: Trim 1.5 inches off each asparagus stem. Add these trimmed stems to the cream and milk and boil over medium heat. Strain and let cream cool. Discard trimmed pieces.
Cook onions in 1 tablespoon of vegetable oil in a medium pan or skillet over medium heat until tender. Set onions aside to cool.
8. Place cornstarch in large bowl with 2 tablespoons milk and whisk to form a smooth paste. Whisk in cooked milk and cream, eggs, cheese, salt, pepper, nutmeg, and cayenne until mixture is smooth. Stir in the onions.
9. Arrange the raw asparagus vertically inside the quiche pan laying one next to the other over the pastry if using a rectangular pan and concentrically if using a round pan. Pour the cream and onions mixture over the vegetables. Gently tap pan on countertop so the mixture settles and covers the asparagus and dislodges any air bubbles.
10. Bake until top of quiche is lightly browned 1 hour until pastry edges are golden and the filling jiggles a little when pan is shaken. Use a toothpick to ensure the filling isn't still oozing liquid. Remove pan and let rest about 30 minutes. Custard will continue cooking as it cools.
11. When ready to serve, use sharp pastry knife remove quiche from pan, slide quiche onto serving plate. Cut into wedges and serve.