Preheat the oven to 375°F (190°C).
Roll out the pie crust and fit it into a 9-inch tart pan or pie dish. Trim any excess dough.
Line the crust with parchment paper and fill with pie weights or dried beans. Blind bake the crust for about 15 minutes until it starts to set.
Remove the parchment paper and weights, and bake for an additional 5 minutes until the crust is lightly golden. Remove from the oven and set aside.
In a skillet, cook the diced bacon over medium heat until crispy. Remove the bacon and set aside, leaving the fat in the skillet.
Add the chopped onion to the skillet and cook until it is soft and translucent. Remove from heat.
In a mixing bowl, whisk together the eggs, heavy cream, and milk until well combined.
Stir in the grated Gruyère cheese, cooked bacon, and onions.
Season with nutmeg, salt, and pepper.
Pour the filling mixture into the pre-baked pie crust.
Place the tart pan or pie dish on a baking sheet to catch any spills.
Bake the quiche in the preheated oven for about 30-35 minutes, or until the filling is set and the top is golden brown.
Remove from the oven and let it cool slightly before slicing.
Enjoy your meal!