Instructions

1. Cook the onions and peppers for the enchilada sauce. Heat the olive oil over medium-high heat in a large saucepan until shimmering. Add the onion, bell peppers, cumin, and chili powder and sauté until the onions are softened, about 5 minutes.
2. Add the tomato purée and simmer for 30 minutes. Add the tomato purée and bring to a simmer. Reduce the heat and simmer until the vegetables are soft, about 30 minutes.
3. Purée the enchilada sauce. Remove from the heat. Blend the sauce directly in the pot with an immersion blender or transfer to a regular blender and blend until smooth.
4. Cook the chicken in the enchilada sauce. Return the enchilada sauce to the pot if needed and place over medium heat. Add the chicken and simmer until the chicken is cooked through, about 10 minutes. Remove the chicken and enchilada sauce from the heat. At this point, you can cool and then refrigerate the chicken in the sauce overnight or for up to 5 days.
5. Layer the tortillas, chicken, cheeses, and sauce in the slow cooker. Coat a 4-quart slow cooker with cooking spray. Combine the cheeses together in a medium bowl. Place a tortilla in the bottom of the slow cooker. Top with about a cup of chicken and sauce and spread evenly. Top with another tortilla, then sprinkle with about 1/4 (1/2 cup) of the cheeses. Repeat the layering (tortilla, cheese, tortilla, chicken and sauce, tortilla ... ) until all of it is used up; you should end with a layer of cheese on top).
6. Cook on low for 8 hours. Cover and cook on the LOW setting for 8 hours. The cheese and sauce will be bubbly and the chicken tender. Sprinkle with the cilantro.
7. Serve. Serve scoops of the finished casserole with sour cream and/or finely chopped red onion if desired.