Preheat oven to 350°F. Melt the butter in a dutch oven over medium heat. Add in the onion & sauté for 5 minutes. Stir in the cream of chicken soup, milk, and sour cream.
Turn off the heat and add two cups of shredded cheddar cheese.
Stir until cheese is melted and mixture is smooth. Next, add hashbrowns.
Combine until well incorporated.
Bake in a preheated oven for 45-50 minutes or until golden and bubbly.