In a 1 1/2-quart saucepan, combine chicken broth, onion powder, garlic powder, poultry seasoning, parsley, paprika, and salt and pepper to taste (I like 1/2 teaspoon salt and 1/8 teaspoon pepper). Bring to a simmer.
Whisk together milk and flour until smooth. Pour into saucepan, whisking continuously, until the mixture bubbles and thickens, about 1 minute. Remove from heat.
To make the casserole:
Adjust an oven rack to the middle position and preheat the oven to 350 degrees. Coat a 9-inch by 13-inch baking dish with nonstick spray.
In a large bowl, add cream of chicken soup, hash browns, sour cream, cheddar cheese, and salt and pepper to taste (I like 1 teaspoon salt and 1/2 teaspoon pepper). Stir to combine, then pour into prepared baking dish.
Bake uncovered for 1 hour, then remove from oven. Melt butter in the microwave. In a large bowl, add corn flakes. Drizzle with melted butter, then spread over the top of the casserole in an even layer. Bake uncovered 20 minutes longer, then garnish with parsley if desired.
Cream of chicken soup: 1 can of condensed cream of chicken soup may be substituted for the homemade version written in to the recipe.
Yield: This recipe makes serving for at least 12, more if you have a bountiful buffet.
Storage: Store leftovers covered in the refrigerated for up to 4 days.
Make ahead: Assemble the casserole through step 2 of the "casserole" section (step 4 overall). Cover and refrigerate up to 24 hours in advance. Proceed with the baking instructions as directed in the recipe.
Freezer: Assemble the casserole through step 2 of the "casserole" section (step 4 overall). Cover and freeze up to 2 months. Thaw overnight in the refrigerator, then proceed with the baking instructions and directed in the recipe.