Preheat your oven to 400°F. Remove your pie crust from the fridge and allow it to sit out on the counter for 15 minutes to soften. In the meantime, cook your bacon over medium heat until crispy, and set on paper towels to cool. Chop your onion and add to a pan with the butter. Cook over medium heat until the onions have softened and caramelized. They should look browned, but not burnt, and have a delicious sweetness to them. Once the onions are caramelized, set them aside to slightly cool. Chop your spinach into slices and add them to a pan with a small amount of butter or oil over medium heat. You just want to wilt the spinach, and it will only take a minute or two before they will shrivel up and darken in color.

Take your softened pie crust and roll it into your lightly-oiled, deep quiche pan. Press the crust against the bottom and sides with your fingers. Bake your crust without any filling for 10 minutes (pre-cooking the crust helps you avoid a soggy pie crust later).

While the crust is baking, whisk your eggs, heavy cream, and salt and pepper in a large bowl. Crumble your bacon into the egg mixture and add your cheese, onions, and spinach. Stir until combined and pour into your pie crust. Cover the pan loosely with aluminium foil and set on a baking sheet. Bake for 1 hour or until the sides are set but the middle still moves slightly when you jiggle the pan. Allow the quiche to cool for 10 minutes, remove the sides from the pan (if using a deep quiche pan), and serve.