In a skillet over medium heat, cook the diced sausage until browned. Remove excess grease if needed.
In a large mixing bowl, combine shredded potatoes, frozen diced onions, cooked sausage, shredded cheddar cheese, sour cream, milk, melted butter, salt, and pepper. Mix until well combined.
Transfer the potato mixture into a greased 9×13 inch baking dish or a similar-sized casserole dish, spreading it evenly.
Bake in the preheated oven for approximately 40-45 minutes, or until the top is golden brown and the casserole is cooked through.
Garnish the casserole with chopped green onions before serving.
