Place the oven rack in the center position and preheat the oven to 375°F.
On a lightly floured surface, roll out the pie crust to a 12-inch circle and transfer it to a 9-inch pie pan, gently pressing the crust into the bottom and up the sides of the pan.
Crimp or flute the edges of the crust if desired. If using a store-bought crust, follow the baking directions on the package and then skip ahead to adding the filling. If using a homemade crust, continue below.
Line the inside of the crust with parchment paper and fill with pie weights (dried beans work well here too). This helps prevent the crust from puffing or shrinking.
Bake the crust for 15–20 minutes, or until it turns a light golden brown.
While the crust is baking, in a medium bowl, combine eggs, whole milk, heavy cream, kosher salt, and ground black pepper. Whisk until fully combined.
When the crust is finished baking, remove it from the oven and reduce the oven temperature to 350°F.
Evenly sprinkle the crumbled bacon and shredded Gruyère cheese into the warm crust. Pour the custard mixture evenly over the filling.
Bake for 30–35 minutes, or until the top is lightly golden and the center is mostly set. To test doneness, insert a knife about 1 inch from the edge; it should come out clean.
Let the quiche cool for 10 minutes before slicing. Garnish with chopped fresh parsley before serving, if desired.