Place the thawed hash browns in a clean kitchen towel or several layers of paper towels.
Squeeze out as much excess liquid as possible from the hash browns.
Transfer the drained hash browns to a large mixing bowl.
Add the shredded white cheddar cheese, salt, and cracked black pepper.
Stir until the hash browns, cheese, and seasonings are evenly combined.
Lightly grease a 9-inch springform pan with cooking spray.
Press the hash brown mixture evenly into the bottom and up the sides of the prepared pan.
Ensure the crust is firmly packed and that there are no holes or gaps, especially along the bottom edges, to help prevent the filling from leaking.
Place the pan in the freezer and freeze the crust for at least 1 hour.
Preheat the oven to 425°F.
Place the frozen hash brown crust in the preheated oven.
Par-bake the crust for 15 minutes.
Remove the crust from the oven and allow it to cool slightly.
While the crust cools, melt the unsalted butter in a skillet over medium heat.
Add the thinly sliced onion.
Cook the onion for 10 to 15 minutes, stirring occasionally, until lightly caramelized.
Season the caramelized onion with salt and black pepper.
Remove the onion from the heat and set aside.
In a large mixing bowl, combine the lightly beaten eggs and heavy cream or whole milk.
Whisk until the mixture is smooth and well combined.
Add the crumbled bacon, shredded white cheddar cheese, sliced chives, and caramelized onion.
Stir until all filling ingredients are evenly distributed.
Season the filling with salt and black pepper to taste.
Pour the egg filling into the par-baked hash brown crust.
Spread the filling evenly inside the crust.
Reduce the oven temperature to 375°F.
Place the filled quiche in the oven.
Bake for 25 to 30 minutes, or until the eggs are just set.
Remove the quiche from the oven.
Carefully run a paring knife around the perimeter of the springform pan.
Loosen and remove the sides of the springform pan.
Return the quiche to the oven for an additional 10 to 15 minutes to allow the sides of the hash brown crust to crisp.
Remove the quiche from the oven.
Allow it to cool for 10 minutes before slicing.
Slice the quiche into wedges.
Garnish with additional sliced chives.
Serve warm.