Prep the pan & brown the sausage: Preheat your oven to 375°F (190°C). Grease a 9x13-inch ceramic or glass baking dish with butter or cooking spray. In a large skillet over medium-high heat, cook the breakfast sausage, breaking it into bite-sized crumbles with a wooden spoon for 6 to 8 minutes until browned. Drain excess fat on a paper towel-lined plate.

Layer the hashbrowns & savory filling: Ensure thawed shredded hashbrowns are pressed firmly between paper towels to remove moisture (this guarantees a crispy, non-soggy bake). Spread the dried hashbrowns evenly across the bottom of the prepared baking dish. Scatter the browned sausage, 2 cups of shredded sharp cheddar, Monterey Jack, and the sliced white parts of the green onions over the potatoes.

Whisk the seasoned custard: In a large mixing bowl or 4-cup measuring pitcher, vigorously whisk together the 8 eggs, half-and-half, kosher salt, cracked black pepper, garlic powder, onion powder, smoked paprika, and dry mustard until completely smooth and emulsified.

Pour & soak: Slowly pour the egg mixture evenly over the hashbrown and sausage layers. Gently press down with a spatula so the custard seeps into every crevice of the shredded potatoes. (Make-ahead note: You can cover tightly and refrigerate overnight at this stage).

Bake to golden perfection: Scatter the remaining 1/2 cup of sharp cheddar over the top. Bake uncovered at 375°F for 45 to 50 minutes, or until the center is puffed and set (a knife inserted into the middle comes out clean) and the top is bubbly and golden-brown. Let rest for 10 minutes, garnish with fresh green onion tops, slice into generous squares, and serve warm!