Preheat oven to 400 degrees.

Add 1 tablespoon butter to a sauce pan over medium heat and then add the onion. Cook until translucent. Add remaining butter. Once melted, add flour and whisk to combine.

Add the milk to the roux mixture. Let cook over medium heat, until thickened, about 5-6 minutes, stirring frequently.

Once thickened, add chicken bouillon and stir well. Add the remaining seasonings. Turn heat off and set aside to cool.

While cooling, pour the frozen shredded potatoes into a large bowl and stir in the sour cream.

Add cheese to the cooled milk mixture and stir well. Pour cheese mixture over the potatoes and then stir. Spread potatoes into a 9 by 13 casserole dish. Top with corn flakes and place in the preheated oven for 40-45 minutes, or until golden brown and bubbling. Serve immediately.