How to Make It
Preheat oven to 375°F. Cook bacon in a 10-inch cast-iron skillet over medium until browned and almost crisp, but still tender, about 6 minutes. Transfer bacon to a plate lined with paper towels. Reserve drippings in skillet, and let cool slightly. Add asparagus to skillet, and cook over medium-high, stirring often, until lightly browned and just beginning to soften, about 3 minutes. Using a slotted spoon, transfer asparagus to a medium bowl.
Add hash browns, bacon, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper to hot drippings in skillet over medium-high; stir to combine. Using a wooden spoon, spread mixture into an even layer across the bottom and 1 inch up sides of skillet. Transfer to preheated oven, and bake until lightly golden, about 30 minutes.
Whisk together eggs, half-and-half, Gruyère, Dijon, and remaining 1 teaspoon salt and 1/2 teaspoon pepper in a large bowl. Stir in scallions.
Remove skillet from oven, and scatter asparagus over crust. Pour egg mixture over asparagus, and return to oven.
Bake until egg mixture is just set, 20 to 25 minutes more. Let cool 5 minutes before slicing.