Preheat oven to 350 degrees and spray a 9" x 13" casserole dish with cooking spray. Place the chicken breasts into the casserole dish and season with salt and pepper. Bake the chicken for about 20 minutes (depending on thickness). Remove chicken from the oven, shred into bite-sized pieces and set aside. (You will use the casserole dish again, so discard any liquid from the chicken and re-coat with cooking spray. There is no need to wash the dish.)

While the chicken is baking, place a large skillet over medium-high heat and melt the butter. When the butter is melted, add the onion, bell pepper and jalapeño. Cook, stirring frequently for about 4–5 minutes, until the onion becomes translucent and the pepper begins to soften. Sprinkle the flour over the onion mixture and stir to combine. Allow the flour to cook for about 1 minute before adding in the milk.

Stir to completely dissolve the flour in the milk and bring the mixture to a bubble. Reduce the heat to medium-low and stir in the sour cream, diced tomatoes, green chiles, salt, chili powder, paprika and garlic powder. Remove from heat and stir in a cup of cheddar cheese. (Reserve the remaining ½ cup for the topping.) When the cheese is melted, fold in the shredded chicken.

Crumble the entire bag of corn chips into coarse crumbs and add about ¾ of the crumbs into the bottom of the casserole dish and spread evenly. Pour the chicken mixture on top of the crumbs and bake 20–25 minutes or until hot and bubbly.

Remove the casserole from the oven and sprinkle the remaining ½ cup of shredded cheese and remaining crushed chips on top. Place the casserole back into the oven for 5 additional minutes, just until cheese is melted. Remove from the oven and let stand for 5 minutes before sprinkling with sliced green onions and serving.