Heat the oven to 400°F. Cook the bacon until crisp, drain it well on paper towels, crumble it, and reserve about 1 tablespoon of bacon fat if you plan to sauté the onion in it.

Sauté the finely chopped onion in the reserved bacon fat over medium heat until softened, about 3 to 5 minutes. Let the onion cool briefly so it does not overheat the custard.

Place the bacon, softened onion, and shredded Swiss cheese in the unbaked 9-inch pie crust. Set the crust on a rimmed baking sheet to make it easier to move and to catch any spills.

Whisk the eggs, half-and-half, salt, pepper, and optional pinch of nutmeg or cayenne until smoothly blended. Pour the custard over the fillings, stopping when it comes close to the top of the crust without spilling.

Bake at 400°F for 10 minutes, then reduce the oven temperature to 350°F. Continue baking for 25 to 35 minutes, until the center is set but still has a slight wobble.

Remove the quiche from the oven and cool it for 10 to 15 minutes before slicing. The custard will continue firming as it rests, making cleaner slices and a more tender texture.