Cook sausage: Preheat oven to 350°F. Lightly coat a 13- x 9-inch baking dish with cooking spray. Heat a large skillet over medium-high. Add sausage, and cook, stirring often to crumble, until browned, about 7 minutes. Transfer to a paper towel-lined plate. Do not wipe skillet clean.

Cook bell pepper: Add bell pepper to skillet, and cook over medium, stirring often, until tender, 3 to 4 minutes. Transfer to plate with sausage.

Make egg mixture: Whisk together eggs, half-and-half, onion powder, 1 teaspoon of the garlic salt, and ½ teaspoon of the black pepper in a bowl until fully combined; stir in sausage, bell pepper, Cheddar cheese, and pepper Jack cheese.

Toss hashbrowns with remaining 1 teaspoon garlic salt and ½ teaspoon black pepper in prepared baking dish; spread in an even layer. Pour egg mixture over hashbrowns. Press to fully submerge hash browns in an even layer.

Bake breakfast casserole: Bake in preheated oven until eggs are set in center and hash browns are golden brown, about 45 minutes. Let stand 15 to 20 minutes before serving. Sprinkle with scallions.