Heat the avocado oil or olive oil in a skillet over medium heat. Add the chopped onion, garlic, cumin, and kosher salt. Cook for about 5 minutes, stirring occasionally, until the onion is softened and lightly browned.
Transfer the onion mixture to the slow cooker.
Add the enchilada sauce, sour cream, and ¼ cup water to the slow cooker. Stir until the sauce is well combined.
Stir in the rinsed and drained black beans until evenly coated in the sauce.
Nestle the chicken thighs or chicken breasts into the sauce mixture.
Cover and cook on LOW for 6–7 hours or HIGH for 3–4 hours, until the chicken is tender and cooked through.
Using two forks, shred the chicken directly in the slow cooker and stir it into the sauce.
Add the tortilla strips and stir them into the mixture so they absorb the enchilada sauce.
Sprinkle the shredded cheese evenly over the top.
Cover the slow cooker and cook for an additional 30 minutes, or until the cheese is melted.
Garnish with cilantro, jalapeños, avocado, or sour cream before serving. Serve warm, optionally over cilantro lime rice.