Preheat oven to 375°F. Unroll the pie crust and place in a 9-inch pie dish. Flute the edges or press with a fork to crimp the edges.

Prick the bottom of the pie crust with a fork. Place a piece of parchment paper on the bottom of the pie crust and fill with pie weights (dried beans work, too!). Bake the crust for 10 minutes in the preheated oven. This is called par-baking the crust.

Meanwhile, place the bacon in a large skillet over medium heat. Cook until crisp, flipping once, about 6–8 minutes. Remove the bacon from the pan and place on a paper towel lined plate to drain. When the bacon has cooled, crumble it gently with your fingers.

Add the butter to the bacon fat. Add the onions, plus ½ teaspoon salt. Cook for 2–3 minutes or until the onions start to soften. Lower heat to medium low and cover the pan. Continue to cook the onions, uncovering and stirring every 2–3 minutes until the onions are medium brown and very soft—about 25 minutes. Remove from the heat and let cool for 10 minutes.

Meanwhile, place the eggs, half & half, remaining ½ teaspoon salt, pepper, and nutmeg in a mixing bowl. Whisk to combine.

Place the cooled onions in the bottom of the par-baked pie crust. Top with the bacon. Pour the egg mixture over the top and top with the shredded Gruyere.

Bake on the middle rack in the preheated oven until the quiche is beginning to turn light brown in spots, about 45–50 minutes. Let cool for 10 minutes before serving or cool completely and serve at room temperature.