Preheat the oven to 350°F (175°C).
In a large skillet over medium-high heat, melt the butter.
Add the diced onion and celery and sauté, stirring occasionally, until translucent and softened, about 8–10 minutes.
Add the minced garlic and cook 1–2 minutes, then season with 1 tsp salt, 1/4 tsp black pepper, cumin, and oregano.
Sprinkle the flour over the vegetables and cook, stirring, for 2–3 minutes to remove the raw flour taste.
Whisk in the chicken broth and milk and bring the mixture to a low boil, whisking constantly, until the sauce thickens.
Remove the skillet from heat and stir in the sour cream and the optional additional 1 tsp salt if using; adjust seasoning to taste.
Spoon a small amount of the sauce into the bottom of a 9×13 casserole dish to prevent sticking.
Place a layer of torn corn tortilla pieces in the dish, then add a layer of shredded chicken, followed by a layer of the cream sauce and a layer of shredded Colby cheese.
Repeat the layers (tortillas, chicken, sauce, cheese) until all ingredients are used, finishing with the remaining cheese on top.
Bake uncovered for 30–35 minutes, or until the casserole is hot and bubbly and the cheese is melted.
Let the casserole rest 5–10 minutes before serving and garnish as desired (sour cream, avocado, cilantro, green or red onion suggested).