Preheat the oven to 375°F (190°C). Fit the pie dough into the pie plate, crimp the edges, and 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, remove the weights and parchment, and bake for another 5 to 8 minutes until the bottom looks dry and just barely set. Set it aside.

Cook the diced bacon in a skillet over medium heat for 6 to 7 minutes, until crisp. Use a slotted spoon to transfer the bacon to a paper towel-lined plate, leaving about 1 tablespoon of fat in the skillet. Add the onion and cook for 4 to 5 minutes, until softened. Let it cool slightly.

In a large bowl, whisk together the eggs, heavy cream, milk, nutmeg, salt, and pepper until smooth and well combined.

Scatter the cooked bacon, sautéed onion, and 1 cup of the Gruyere evenly over the bottom of the pre-baked crust. Pour the egg custard slowly and evenly over the filling. Sprinkle the remaining 1/2 cup Gruyere on top.

Bake the quiche for 30 to 38 minutes, until it is puffed and golden brown on top. The edges should be set, and the center should jiggle slightly.

Let the quiche cool on a wire rack for at least 15 to 20 minutes before slicing. Garnish with chopped chives if desired.