Preheat oven to 350°F. Spray a 9″ X 13” casserole dish with non-stick cooking spray.
Heat butter and oil in Dutch oven over medium heat. Add onion, bell pepper, and jalapeno; cook for 7-10 minutes or until tender. Stir often. Add garlic and cook for 30 seconds.
Stir in flour and spices to vegetable mixture. Cook for one minute. Add chicken broth and milk. Turn heat to medium-high and cook until sauce thickens, stirring constantly. Remove sauce from heat and stir in diced tomatoes, mushrooms and chicken.
Layer 5 corn tortillas on bottom of casserole dish, top with one-half of sauce and one-third of cheese. Repeat layering of tortillas, sauce, and cheese. Top with corn tortillas and remaining shredded cheese.
Cover with foil, and bake at 350°F for 30 minutes. Remove foil and bake for an additional 5 minutes or until casserole is bubbly and cheese is melted.