Preheat oven to 450°F. Press hash browns between a few paper towls or a cheesecloth to drain as much excess moisture as possible. In a 9″ pie plate, mix together the hash browns and melted butter. Season with Kosher salt and freshly ground pepper. Gently press the hash browns into the bottom and up the sides to form a thin crust. Bake for 20 to 25 minutes until the edes are golden brown and are starting to crisp. Remove from oven and lower temperature to 350°F.
Meanwhile, cook the chopped bacon in a medium skillet over medium-low heat until crisp. Drain on a paper-towel lined plate. Remove all but 1 tablespoon of the bacon grease from the skillet and reserve (keep refrigerated) for another use.
Squeeze spinach in between a few paper towels to remove excess moisture.
Add onion to the skillet with the 1 tablespoon of bacon grease, season with Kosher salt, and cook for 4-6 minutes, stirring occasionally. Add garlic and cook for 1 minute, stirring constantly to avoid burning. Add spinach and cook just until mixture is warmed through, about 1 minute.
In a medium mixing bowl, lightly beat the eggs, then stir in half and half, cheese, bacon, and spinach mixture. Season with Kosher salt and freshly ground pepper. Pour egg mixture over hashbrown crust and cook in 350°F oven for 30 to 35 minutes, until quiche is light golden brown on top.
Transfer to a cooling rack and let cool for at least 10 minutes before slicing and serving. Alternatively, let cool completely, refrigerate, and reheat in a 350°F oven.