Preheat oven to 350° F.
In a large skillet over medium-high heat, melt butter.
Add onion and celery and sauté (stirring occasionally) for about 8-10 minutes or until veggies are translucent and softened.
Add garlic and cook for 1-2 minutes. Season with salt, pepper, cumin and oregano.
Sprinkle in flour and cook, stirring, for 2-3 minutes.
Whisk in chicken broth and milk and bring to a low boil until mixture is thickened, whisking constantly.
Turn off heat and stir in sour cream.
Add a bit of the sauce to the bottom of a 9x13 casserole dish.
Place a layer of ripped corn tortillas in the dish.
Next, add a layer of shredded chicken.
Cover chicken completely with a layer of the cream sauce.
Add a layer a cheese on top of the sauce.
Repeat these layers: tortillas, chicken, sauce, cheese until all ingredients are in dish. Final layer should be the last of the cheese.
Bake for 30-35 minutes or until hot and bubbly. Let rest for 5-10 minutes.
Garnish with sour cream, avocado, cilantro, green or red onion or whatever your heart desires and serve!