Cook the chicken.
Place the chicken breasts in the bottom of a 4–6-quart crockpot. Season both sides with salt, pepper, garlic powder, onion powder, and cumin.
Sprinkle the diced onion over the chicken, then add the pats of butter.
Cover and cook on HIGH for approximately 3 hours or LOW for approximately 6 hours, until the chicken reaches an internal temperature of 165°F in the thickest part. Cooking time can vary depending on the size of the chicken breasts and your slow cooker.
Prepare the baking dishes.
For enchiladas you plan to bake right away, preheat the oven to 350°F. Spray two 9×13-inch casserole dishes with nonstick cooking spray.
If you plan to freeze a pan, follow the freezing directions below before assembling that portion.
Make the filling.
Shred the chicken in the crockpot using two forks or a shredding tool, mixing it with the cooking juices.
Add 1½ cups of shredded Mexican cheese, one can of red enchilada sauce, the drained black beans, and the drained green chiles. Stir until everything is evenly combined.
Fill and roll the tortillas.
Divide the filling evenly among the 20 flour tortillas, placing a line of filling down the center of each one.
Roll each tortilla around the filling and place it seam side down in a prepared baking dish. Divide the enchiladas between the two pans, arranging them to fit.
Add sauce and bake.
Pour one can of enchilada sauce over each pan, spreading it across the tortillas.
Cover with aluminum foil and bake for 15 minutes, or until heated through.
Remove the foil and sprinkle each pan with ¾ cup of shredded cheese. Bake uncovered for approximately 10 more minutes, until the cheese is melted and bubbly.
These baking times are for freshly assembled enchiladas with warm filling. Refrigerated or thawed enchiladas need additional time.
Add toppings and serve.
Serve warm with sour cream, olives, pico de gallo, shredded lettuce, or your favorite toppings.