In medium sized saucepan, over medium heat, melt 2 tablespoons butter. Once butter is melted add 2 tablespoons flour and all seasonings. Whisk to create a paste. Add milk, whisking as you pour. Whisk until completely combined.
In a quart sized mason jar combine 2 tablespoons flour and chicken broth. Seal tightly and shake the living tar out of your mixture. You will create a smooth flour mixture for thickening your sauce called a slurry.
Slowly pour slurry into mixture, whisking constantly. Bring to a boil and cook 2 minutes, stirring constantly.
Pour soup into 9x13 casserole dish. Add remaining 6 tablespoons butter and onions. Stir until butter is melted. This will also start to cool your soup. Add cheese and frozen hash browns. Mix well with a spoon.
Bake 25 minutes or until the top is golden brown.
Serve and enjoy!
Notes
If desired you can sprinkle the top of the casserole with additional cheese for the last 10 minutes of cook time.You can also divide this recipe into 2 8×8 casserole dishes.