Preheat your oven to 375°F (190°C). Roll out the pie dough and gently press it into a 9-inch tart pan or pie dish. Prick the bottom all over with a fork.

Line the crust with parchment paper and fill it with pie weights or dried beans. Bake for 15 minutes, then remove the weights and parchment and bake for another 5-7 minutes, until the bottom is just starting to turn golden. Set aside to cool slightly.

While the crust bakes, place the bacon pieces in a cold skillet. Turn the heat to medium and cook, stirring occasionally, until the fat is rendered and the bacon is crispy but not burnt. Use a slotted spoon to transfer the bacon to a paper towel-lined plate.

Pour off all but 1 tablespoon of the bacon fat from the pan. Add the diced onion to the pan and cook over medium heat for 3-4 minutes, until softened and translucent. Scrape the onions into the same bowl as the bacon and let cool.

In a large mixing bowl, whisk the eggs until they are completely broken up and frothy. Slowly pour in the heavy cream while whisking constantly. Add the salt, pepper, and freshly grated nutmeg. Whisk until the mixture is completely smooth and homogenous.

Spread the cooled bacon and onion mixture evenly over the bottom of the pre-baked crust. Sprinkle the shredded Gruyère cheese over the top.

Slowly and carefully pour the egg and cream custard over the cheese and bacon. The custard should come almost to the top of the crust. Give the pan a gentle tap on the counter to release any air bubbles.

Place the quiche on a baking sheet and bake at 375°F (190°C) for 35-40 minutes. The quiche is done when the center is just set but still jiggles slightly when you gently shake the pan. The top should be golden brown and puffed. A knife inserted near the center should come out clean.

Let the quiche cool on a wire rack for at least 15-20 minutes before slicing. This resting period allows the custard to fully set, making for perfect, clean slices.