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