Preheat the Oven: Preheat your oven to 375°F (190°C). Roll out the pie crust and press it into a 9-inch pie dish. Trim any excess and prick the bottom with a fork to prevent it from puffing up. Partially bake the crust for 10 minutes, then set aside.
Cook the Bacon and Onions: In a skillet, cook the diced bacon over medium heat until crispy. Remove the bacon and drain on paper towels. In the same skillet, sauté the chopped onions in the bacon fat until soft and golden, about 5 minutes. Set aside.
Prepare the Filling: In a bowl, whisk together the eggs, heavy cream, and milk until smooth. Stir in the shredded cheese, bacon, and onions. Add salt, pepper, and a pinch of nutmeg (if using) for extra flavor.
Assemble the Quiche: Pour the egg mixture into the pre-baked pie crust, spreading it evenly. Place the quiche in the preheated oven and bake for 35-40 minutes, or until the custard is set and the top is lightly golden.
Cool and Serve: Let the quiche cool for about 10 minutes before slicing and serving. It can be enjoyed warm or at room temperature, making it perfect for any time of day.