Preheat oven to 350°F. Spray a 9×13-inch baking dish with cooking spray and set aside.
In a medium pot over medium heat, melt butter. Add garlic and cook for 30 seconds. Whisk in flour and cook for 1 minute, stirring constantly. Slowly add chicken broth while whisking constantly. Increase heat slightly and cook until the sauce boils and thickens—it should leave a clear line when you run your finger down the back of a spoon. Remove from heat and stir in sour cream, green chiles, lime juice, cilantro, cumin, onion powder, garlic powder, and salt and pepper to taste (I add 1/2 tsp each).
Spread 1/2 cup of the sauce on the bottom of the baking dish.
In a large bowl, mix shredded chicken, 1-1/2 cups cheese, and about half the sauce (just eyeball it). Add black beans, if using. Taste and adjust seasoning if needed.
Lay out all 8 tortillas. Divide filling evenly and roll each one tightly. Place seam-side down in the baking dish.
Pour the remaining sauce over the enchiladas and sprinkle evenly with remaining 1-1/2 cups cheese.
Cover with foil and bake for 25 minutes. Uncover and bake an additional 5–10 minutes, or until cheese is melted and bubbly.
Top with desired garnishes and serve warm.
