In a large pot over medium heat, add the butter and onion. Cook until the onion is softened and translucent, about 5-6 minutes.Add the flour. Cook, stirring constantly, for about a minute.Add the chicken broth and milk to the mixture. Add each one slowly while whisking.Add the salt, pepper and thyme and stir to combine.Bring to a boil and then reduce then simmer, stirring frequently, until the mixture is slightly thickened, about 5 minutes.Remove from the heat and stir in the cheese until smooth.Fold in the thawed hash browns until well combined.Then in the sour cream. Spread the mixture into a 9 x 13 inch baking dish.Combine the topping ingredients and sprinkle over the mixture.Bake at 325 for 45 minutes, until hot and bubbly.