Preheat the oven to 375°F. Spray a 1½-quart baking dish with nonstick spray; set aside.
Melt 1½ tablespoons of butter. In a small bowl, combine the melted butter, Panko breadcrumbs, and Parmesan cheese; set aside.
Melt the remaining 1½ tablespoons of butter in a medium saucepan over medium heat. Add the flour and cook, stirring constantly, for 1 minute.
Slowly whisk in the milk. Cook, whisking frequently, until the sauce is slightly thickened, about 3 to 4 minutes.
Whisk in the Dijon mustard, garlic powder, salt, and pepper. Remove the pan from the heat. Add ¾ cup of the white cheddar and whisk until smooth. Stir in the sour cream until fully incorporated.
Add the chicken and broccoli; stir to combine.
Transfer the chicken-and-broccoli mixture to the prepared baking dish. Sprinkle the remaining ¾ cup of white cheddar over the chicken and broccoli, then top with the Parmesan-Panko mixture. (See Tip 3)
Bake uncovered at 375°F for 25 to 30 minutes, until hot and bubbly and the top is golden brown.
Let the casserole rest for about 5 minutes before serving. Garnish with parsley, if desired.
Yield: 2 to 4 servings. (See Tip 4)