Preheat the oven to 450°. Roll out the pastry shell to about 11" in diameter. Gently place the pastry into a 9" pie pan and press it into the pan pushing on the sides so they don't slip while baking. Fold any overhanging pastry under and press the edges into a decorative pattern with your thumb and forefinger or by using the tines on a fork. Prick the bottom of the shell several times with the fork. Place a piece of parchment paper over the pastry and fill with pie weights or one pound dried beans. Bake for 8-9 minutes until pastry is set. Remove from oven and set aside.
Turn the oven down to 350°.
Fill a medium bowl with cool water and ice. Set aside.
Bring a shallow pot of water to a boil. Trim the asparagus spears, getting rid of the tough bottom of the stem and reserving the top portion in 2-3 inch pieces. With the middle portion of the stem, slice them into ¼" rounds. When the water comes to a boil, transfer the asparagus to the boiling water for two minutes to blanche it.
Use a slotted spoon or spider to transfer the asparagus to the ice bath to stop the cooking and shock it. Set aside.
Add two teaspoons of butter to a pan with a sprinkle of kosher salt and saute the mushrooms for 3-5 minutes over medium high heat or until tender and lightly brown. Transfer mushrooms to a small bowl.
Add the remaining two teaspoons butter to the same skillet and saute the leeks over medium heat until they soften and become translucent, about 5 minutes. If you have a lid for the pan, place the lid on for a minute or so to assist in softening the vegetable (this has a steaming affect). When the leeks are softened, remove the pan from the heat and add the mushrooms back to the skillet.
Drain the asparagus and pat dry with paper towels. Reserve the tips. Add the asparagus rounds and gruyere cheese to the mushroom mixture. Stir to combine.
In a medium bowl, whisk the eggs until frothy. Add the half and half, milk, salt, pepper and nutmeg. Whisk to combine. Add the vegetable mixture to the eggs and stir until well combined. Pour the filling into the pie shell.
Place the dish on a baking sheet and set in the hot oven. Place a piece of tin foil over the top of the quiche and bake for 45 minutes. Remove the foil and continue to bake for another 20-25 minutes or until the quiche is set. Remove from oven and let stand at least 10 minutes before serving.
Quiche can be made a day ahead of time, covered and stored in the refrigerator and reheated in a 300° oven for 20-25 minutes.
Notes
Quiche is best the first day, but leftovers can be kept refrigerated for 2-3 days.Reheat leftovers in a low oven (about 325°) for 10-12 minutes until warm.