In a large skillet over medium-high heat, cook the breakfast sausage and diced yellow onion, breaking the meat into small crumbles with a wooden spoon for 7 to 8 minutes until thoroughly browned and cooked through. Stir in the minced garlic for 60 seconds until fragrant.

Remove from heat, drain any excess rendered fat, and set aside to cool slightly.

Preheat your oven to 375°F (190°C). Generously grease a 9x13-inch ceramic or glass baking dish with non-stick butter spray.

Spread the thawed, patted-dry shredded hashbrowns evenly across the bottom of the dish, pressing down gently to form a sturdy base layer.

Scatter the savory browned sausage and onions evenly over the hashbrowns, then sprinkle with 2 cups of the shredded sharp cheddar and Monterey Jack cheese.

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

Slowly and evenly pour the egg custard mixture all over the casserole dish, allowing it to soak deep into the crevices between the hashbrowns and sausage.

Top the casserole evenly with the remaining 1 1/2 cups of shredded cheese.

Bake uncovered at 375°F (190°C) for 45 to 50 minutes, until the center is completely set (a knife inserted into the middle comes out clean) and the cheese on top is puffed, bubbling furiously, and toasted golden brown.

Remove from oven and let the casserole rest on a wire rack for 10 minutes. This resting period allows the egg custard to set firmly for clean, neat bakery-style square slices.

Scatter freshly sliced green scallions over the top, slice into 10 squares, and serve warm with fresh fruit and hot coffee!