Preheat your oven to 350℉ (177℃) and lightly spray a 9x13 baking dish with cooking spray.
In a large skillet melt 2 tablespoons of the butter over medium heat. Add the onion, and bell peppers. Sauté until softened, about 5 minutes.
2 Tablespoons salted butter, 1 medium yellow onion, 1 medium green bell pepper, 1 medium red bell pepper
Add the remaining 6 tablespoons of butter and melt. Add the flour and whisk for 1-2 minutes. Slowly pour the broth in while whisking followed by the milk. Add the diced tomatoes, green chilies, and all the seasonings. Whisk until the sauce thickens and starts to simmer.
6 Tablespoons salted butter, ½ cup all-purpose flour, 1 ½ cups chicken broth, 1 ¼ cups milk, 1 (14.5-ounce) can petite diced tomatoes, 1 (4-ounce) can diced green chilies, 1 ½ teaspoons kosher salt, 1 ½ teaspoons garlic powder, 1 ½ teaspoons onion powder, 1 teaspoon celery salt, 1 teaspoon ground cumin, 1 teaspoon chili powder, ½ teaspoon smoked paprika, ½ teaspoon coarsely ground black pepper
Remove from the heat and add the sour cream. Whisk until smooth. Remove 1 ½ cups of this mixture to a separate bowl and set aside.
½ cup sour cream
Add the chicken and corn to the pot and stir until combined.
3 cups cooked and shredded chicken, 1 cup frozen corn
Spread half of the chicken mixture on the bottom of the baking dish. Layer ⅓ of the cheese on top followed by ½ of the tortillas in an even layer. Spread out the remaining chicken mixture followed by ⅓ of the cheese and the remaining tortillas. Spread the reserved sauce over top of the tortillas and top with the remaining ⅓ of the cheese.
3 cups freshly grated sharp cheddar cheese, 12 (5-inch) corn tortillas
Bake for about 30 minutes or until the edges are bubbling and the cheese has melted and starting to brown very slightly.
Let sit for 10-15 minutes before serving. Garnish with chopped cilantro, avocados, and sour cream, if desired.
Chopped cilantro, avocado, or sour cream for garnish