Mix together your filling ingredients (cream cheese, chicken, and black beans, and half of the green chiles). Set aside.
In a small sauce pan, melt the butter. If using an onion, add this to the butter and saute until softened. If using onion powder, add this to the butter. Add in the flour and make a roux by whisking together. You'll want it to come together into a smooth paste like texture.
Slowly add in the stick, milk, and enchilada sauce small amounts at a time while constantly whisking. You'll want the sauce to remain thickened and smooth. Bring to a simmer and add in the salt, pepper, remaining green chiles, and sour cream, stirring until combined.
Spray a 9x13 casserole dish with nonstick spray.
Layer a small amount of sauce across the bottom of the dish, just covering the bottom.
Fill each tortilla with about 2-3 tablespoons of filling and place seam side down.
Pour the remaining sauce over the top of the enchiladas and sprinkle with cheese.