Preheat oven to 375°F (190°C). Grease a 9x13-inch ceramic or glass casserole baking dish thoroughly with nonstick cooking spray or softened butter.

Heat a large skillet over medium-high heat. Add ground breakfast sausage, breaking it into bite-sized crumbles with a wooden spatula. Sauté for 6 to 8 minutes until golden-brown and fully cooked through. Drain excess rendered fat and set aside on a plate.

Spread thawed, dry shredded hashbrowns evenly across the bottom of the prepared 9x13 baking dish.

Evenly scatter the cooked sausage crumbles, 1 ½ cups of shredded sharp cheddar cheese, 1 cup of Monterey Jack cheese, and the sliced whites of the green onions over the hashbrown layer. Gently toss together with a fork so the fillings intermingle.

In a large mixing bowl, vigorously whisk together eggs, whole milk, heavy cream, garlic powder, smoked paprika, onion powder, kosher salt, and black pepper until completely combined and frothy.

Pour egg custard slowly and evenly over the hashbrown and sausage mixture, ensuring all corners are saturated.

Sprinkle remaining ½ cup of sharp cheddar cheese across the top. Bake uncovered at 375°F for 35 to 40 minutes, until the center is puffed and set, and the cheese is bubbling with golden-brown edges.

Let rest for 10 minutes to allow the layers to settle cleanly. Garnish with reserved green scallion tops, slice into 8 generous squares, and serve warm!