How to Make It
Preheat oven to 350°F; mist a 9-by-13-inch baking dish with cooking spray. Bring a pot of salted water to a boil. Cook pasta until al dente, about 10 minutes or as package directs. Drain.
Melt butter in a Dutch oven over medium-high heat. Add bell pepper, onion, garlic, cumin, chili powder and 2 tsp. salt; sauté until onion is tender, 6 to 8 minutes. Add tomato paste and cook, stirring constantly, 2 minutes longer.
Whisk in flour until smooth. Cook, whisking constantly, for 1 minute. Gradually whisk in milk. Cook, whisking, until thickened, about 5 minutes. Add diced tomatoes and 1 cup cheese. Cook, whisking, for 1 minute or until cheese melts.
Remove from heat. Stir in chicken and pasta. Scrape into baking dish; sprinkle with remaining cheese. Bake until bubbly, 20 to 25 minutes. (Preheat broiler to high and broil casserole for a minute to brown, if desired.) Let stand for 10 to 15 minutes. Sprinkle with cilantro, if desired; serve.