Preheat oven to 350°F and grease a 9x13-inch baking dish with butter or cooking spray.
In a large skillet over medium heat, cook the breakfast sausage with diced onion, breaking up the meat with a spoon, until sausage is browned and cooked through, about 8-10 minutes, then drain.
In a large mixing bowl, combine cream of chicken soup, sour cream, melted butter, garlic powder, paprika, salt, and pepper, stirring until smooth.
Add thawed hash browns, cooked sausage mixture, and 2 cups of shredded cheddar cheese to the soup mixture, stirring until everything is evenly combined.
Spread the mixture into the prepared baking dish and top with remaining 1 cup of cheddar cheese.
Bake uncovered for 40-45 minutes until the casserole is hot and bubbly and the top is golden brown.
Let cool for 10 minutes before serving, then garnish with sliced green onions.