Impossible Breakfast Casserole

1 lb. pork sausage
1 small onion, chopped
2 1/2 cups frozen hash browns
1 cup shredded cheddar cheese
1/4 cup chopped green onion
1 3/4 cups milk
1 cup Bisquick
4 eggs
1/4 teaspoon salt
1/8 teaspoon pepper
1/2 cup shredded cheddar cheese

Brown sausage and onion. Drain and mix with all other ingredients except the last listed cheese. Mix and place in a large casserole dish. Top with cheese. Bake for 40 minutes at 400 degrees