Preheat your oven to 350°F (175°C).
In a large bowl, mix the thawed hashbrowns with melted butter.
Add the chopped onion and stir until evenly combined.
In a separate bowl, mix sour cream and cream of chicken soup.
Add the sour cream mixture to the hashbrowns and stir well.
Fold in the shredded cheddar cheese, reserving some for topping.
If using, add the crumbled bacon and mix thoroughly.
Transfer the mixture to a greased 9x13-inch baking dish.
Sprinkle the reserved cheese on top of the casserole.
Bake for 45 minutes, or until golden brown and bubbly.