• Preheat the Oven

Preheat the oven to 375°F.

Lightly grease a 9 × 13-inch casserole dish and set it aside.
 

• Cook the Potatoes and Broccoli

Place the diced potatoes in a large pot and cover them with cold water.

Bring the water to a boil and cook the potatoes for approximately 8–10 minutes, or until they are beginning to soften but are not completely tender.

Add the broccoli florets during the final 2–3 minutes of cooking.

Drain the potatoes and broccoli thoroughly and set them aside. Do not overcook them because they will finish cooking in the oven.
 

• Cook the Chicken

Melt 2 tablespoons of butter in a large skillet over medium heat.

Season the chicken pieces with ½ teaspoon salt and the black pepper.

Add the chicken to the skillet and cook for approximately 8–10 minutes, stirring occasionally, until lightly browned and cooked through. The center of the chicken should reach 165°F.

Transfer the cooked chicken to a large mixing bowl.
 

• Sauté the Onion

Melt the remaining 2 tablespoons of butter in the same skillet.

Add the chopped onion and remaining ½ teaspoon salt. Cook for approximately 5 minutes, stirring occasionally, until the onion is softened and translucent.

Transfer the cooked onion to the bowl with the chicken.

• Prepare the Creamy Cheese Sauce

Add the chicken broth, heavy cream, garlic salt, onion powder, dried dill, paprika, and optional jalapeños to the skillet.

Stir well and heat the mixture over medium-low heat until it is hot and gently steaming. Do not bring it to a rapid boil.

Mix the cornstarch and cold water in a small bowl until smooth.

Slowly pour the slurry into the warm sauce while whisking continuously. Cook for 2–3 minutes, stirring frequently, until the sauce thickens.

Reduce the heat to low and stir in 1 cup of fiesta-blend cheese until completely melted.

Remove the skillet from the heat. Let the sauce cool for 2–3 minutes, then stir in the sour cream until smooth and creamy. Taste and adjust the salt and pepper if needed.

• Assemble the Casserole

Add the drained potatoes and broccoli to the bowl containing the chicken and onions.

Pour the creamy cheese sauce over everything and gently fold until the chicken and vegetables are evenly coated.

Transfer the mixture to the prepared casserole dish and spread it into an even layer.

• Bake the Casserole

Cover the casserole dish with aluminum foil and bake for 15 minutes.

Carefully remove the foil and sprinkle 1½ cups of fiesta-blend cheese evenly over the top.

Return the casserole to the oven uncovered and bake for another 10–15 minutes, or until the sauce is bubbling and the cheese is completely melted.

For a lightly golden topping, broil the casserole for 1–2 minutes at the end. Watch it closely so the cheese does not burn.

• Rest and Serve

Remove the casserole from the oven and let it rest for 5–10 minutes. This allows the creamy sauce to settle and thicken slightly before serving.