Heat the oil in a skillet over medium heat. Add the chopped onion, garlic, cumin and salt and cook, stirring, until the onion has softened and lightly browned (about 5 minutes). This step builds a flavour base before everything goes into the slow cooker.
Transfer the cooked onion and garlic mixture into your slow cooker. Add the enchilada sauce, sour cream, ¼ cup of water, and stir to combine. Then fold in the drained black beans so they’re coated in the sauce, and nestle the chicken thighs (or breasts) into the mix. Let the slow cooker do its job.
Cover and cook on high for 3-4 hours or low for 6-7 hours until the chicken is tender and easily shredded. Use two forks to shred the chicken right in the pot. Then stir in the tortilla strips, top with the shredded cheese, cover, and cook for another ~30 minutes until the cheese melts. Finally, garnish with chopped cilantro, jalapeños (and optional avocado, sour cream or crema) before serving.