Preheat the oven to 375 degrees Fahrenheit and spray a large 9×13-inch casserole dish with non-stick cooking spray.
Heat the oil in a large skillet set over medium heat. Once the pan is hot and the oil shimmers, add the diced onion and bell pepper and cook until softened, about 5 minutes.
Stir in the garlic, chili powder, salt, and black pepper and cook until fragrant, about 30 seconds.
Sprinkle the veggies with all-purpose flour, stirring continuously for about a minute or two until the flour is completely incorporated with the veggies and the flour turns from white to golden brown.
Add the chicken broth, stirring continuously, until the flour is dissolved in the broth, then add the sour cream and whisk until thickened, about 5 minutes (if necessary, reduce the heat to maintain a simmer).
Stir one cup of the shredded cheese and the softened cream cheese until fully melted.
Once the sauce has been prepared, add the cooked and shredded chicken plus the drained can of Ro-Tel tomatoes. Once incorporated, remove from the heat.
To layer the casserole, tear half of the corn tortillas into quarters or long strips and add them to the bottom of the prepared casserole dish.
Spread half of the cheesy chicken sauce over the tortillas in an even layer, then sprinkle with half of the remaining cheese. Repeat with the remaining corn tortillas, cheesy chicken sauce, and shredded cheese.
Transfer to the oven and bake for 25 minutes or until the cheese is melted and bubbly around the edges. Allow it to rest for 5 minutes before serving.