Roll the dough into a 14-inch circle and gently place it into a 9-inch springform pan, pressing it into the corners. Trim any excess dough and refrigerate the crust for at least 30 minutes to prevent shrinking during baking.

In a skillet, heat over medium heat and add the thick-sliced bacon, cooking until crisp and browned, about 8-10 minutes. Remove the bacon and place it on paper towels to drain, leaving behind about 1 tablespoon of drippings in the pan.

In the same skillet with the reserved drippings, add the chopped sweet onions. Cook them over medium-low heat, stirring occasionally, for 20-25 minutes until they are soft and caramelized. Stir in minced thyme, coarsely ground pepper, and nutmeg, then let cool slightly.

Crumble the cooled bacon and mix it with the caramelized onions and shredded Gruyere and Parmesan cheeses in a large bowl. Spoon this filling evenly into the prepared crust.

In a separate bowl, whisk together the eggs, 2 cups of milk, and 1 cup of heavy whipping cream until thoroughly combined. Pour this mixture over the assembled filling in the crust.

Place the assembled quiche on a rimmed baking sheet and bake at 350°F for 75-85 minutes. The quiche is done when the filling is set and slightly puffed, with a golden-brown top. Let it cool for 15 minutes before carefully removing the springform pan.