Heat oil in a skillet over medium heat. Add onion, garlic, cumin, and salt. Cook for 5 minutes until onion softens.
Transfer onion mixture to slow cooker. Stir in enchilada sauce, sour cream, and a splash of water. Add black beans and mix well.
Place chicken in slow cooker, nestling into the sauce. Cover and cook on high for 3–4 hours or low for 6–7 hours until tender.
Shred chicken in the slow cooker with two forks and mix with sauce and beans.
Stir in tortilla strips, then sprinkle cheese on top. Cover and cook another 30 minutes until cheese is melted.
Garnish with fresh cilantro and jalapeños. Serve with avocado, sour cream, or cilantro lime rice.