Instructions:

1. Preheat the oven to 350 degrees F (175 degrees C).

For the Sauce:

1. Heat butter in a large skillet over medium-high heat.
2. Saute onion and bell peppers in butter until softened, about 2 minutes. Add garlic and saute for another 30 seconds.
3. Transfer cooked vegetables to a large bowl.
4. Melt Velveeta cheese in a microwave safe dish. Add to the bowl of vegetables.
5. Add sour cream, both cans of condensed soup, Rotel tomatoes with and green chiles, chicken broth, chili powder, cumin, salt and pepper; mix until sauce is well-combined.

Assembly:

1. Spread about 1/4 cup of sauce in the bottom of a 9x13-inch baking dish.
2. Layer with 1/2 of the tortillas, then the chicken, 1/2 of the sauce, and 1/2 of the Cheddar cheese.
3. Spread the remaining chicken over the tortillas.
4. Spread most of the remaining sauce over the chicken, reserving 1/2 cup of the sauce.
5. Top with another 1/3 of the Cheddar, and the remaining tortillas.

Topping:

1. Spread the remaining 1/2 cup of sauce over the top, and sprinkle with the remaining Cheddar.
2. Bake casserole in the preheated oven until bubbling, about 40-45 minutes.
3. Turn the oven to broil and broil until the top is golden, about 2 to 3 more minutes.