Preheat the oven to 400°F and set a rack in the middle position. Remove the pie crust from freezer and thaw until just soft enough to easily prick with a fork, about 10 minutes. Prick the bottom and sides all over with a fork (pricking about an inch apart). Bake until fully cooked and lightly golden, 10 to 15 minutes. (Keep an eye on it…if it puffs up while cooking, gently prick it with a fork so it will deflate.) Don’t worry if the crust cracks while baking; see my note below on how to fix it before proceeding. Set aside and turn oven down to 325°F.
Heat the butter in a small skillet over medium-low heat. Cook the shallots until soft and translucent, about 8 minutes. Do not brown. Set aside to cool.
In a medium bowl, whisk together eggs, heavy cream, nutmeg, salt and cayenne pepper.
Place the cooked pie crust on a baking sheet (this makes it easy to move in and out of oven). Spread the shallots over the bottom of the cooked crust, then sprinkle the shredded Gruyere over top. Scatter the spinach evenly over cheese, breaking up the clumps as best you can. Pour the egg and cream mixture over top.
Bake at 325°F degrees for 50 to 55 minutes until the custard is set and top is lightly golden. Serve hot or warm.
Note: Don’t panic if your crust cracks – you can easily fix it. Make a smooth paste by mixing 1 tablespoon of flour with 1 tablespoon of water. Use your fingers to patch up and fill any cracks, then place the crust back in the oven for a minute or so to set. It should be good as new.
Make ahead: This quiche can be made up to a day ahead of time and refrigerated. To reheat: Cover the quiche with aluminum foil and bake in a preheated 300°F oven for 35 to 45 minutes, or until hot in the center.
Freezer-Friendly Instructions: The cooked quiche can be frozen for up to 3 months. Remove the quiche from the freezer about 24 hours prior to eating and reheat it, covered with foil, in a 300°F oven until hot in the center.
Note: I usually defrost the frozen spinach quickly by placing it in a fine mesh strainer and running hot water over it. I then gather the spinach into a ball and squeeze it dry. It takes a few minutes of squeezing and re-squeezing to get all the water out. Be patient…you need the spinach completely dry, otherwise your quiche will be watery.