Preheat oven to 350 degrees, with the rack in the lowest position.
In a large skillet, melt butter over medium. Add bacon and cook until starting to brown. Add leek, asparagus, garlic, and mushrooms; season with salt and pepper. Cook, stirring occasionally, until asparagus is crisp-tender, about 6-8 minutes. Let cool.
In a large bowl, whisk together eggs, half and half, 1/2 teaspoon salt and a pinch of nutmeg.
Place pie crust on a rimmed baking sheet. Sprinkle with cheese; top with asparagus mixture. Pour egg mixture on top.
Bake until center of quiche is just set, 50 to 60 minutes, rotating sheet after about 30 minutes. Let stand for 15 minutes before serving.
To store, let cool, then refrigerate, up to 1 day. Reheat at 350 until warm in center, about 30 minutes. It does warm up very well.
To save time I used a pre-made (Maria Calendar) frozen crust and it worked just fine.