1. Preheat oven to 375 degrees and lightly spray a 9×13 casserole pan with nonstick spray.
2. In a medium pan, melt the butter over medium heat then add the garlic to saute for 1 minute. Sprinkle the flour over the butter and whisk to combine. Cook for 1 minute. While whisking, pour in 1 C of chicken broth and allow it to come to a simmer. Add any additional broth if the sauce is too thick. It should be the consistency of gravy. Allow it to come to a simmer again to check the thickness once more. Remove from the heat and add the remaining ingredients for the sauce and mix until all combined.
3. Spread a small amount of the sauce on the bottom of the pan, about 1/2 C. Lay down six tortillas on top of the sauce then add half of the chicken, avocado and 1/3 of the cheese. Drizzle a bit of sauce over the top. Repeat once more. Top the whole dish with 6 more tortillas the spread the remaining sauce over the top and sprinkle on the remaining cheese. Bake for 20-25 minutes until the cheese is bubbling.