Preheat the oven to 500°F.
Heat the oil in a 10-inch oven-safe (preferably cast-iron) skillet over medium heat. Add in the onion, garlic, cumin, and salt and cook, stirring for about 5 minutes.
Transfer the onion mixture to a large bowl; set aside the hot skillet.
Add the enchilada sauce, sour cream, and ¼ cup water to the onion mixture and stir to combine. Fold in the tortillas, chicken, and beans until thoroughly coated.
Add mixture to skillet and place in the oven for 3 minutes so it heats through. Take out of oven and top with cheese. Place back in oven for 5 minutes until bubbly.
Top skillet with sour cream, avocados, cilantro, and jalapeño if you choose.
