Place the chicken breasts and diced onion in the slow cooker. Add the enchilada sauce, minced garlic, cumin, and salt; stir to coat.
Cook on high for 4 hours or low for 6–8 hours, until the thickest piece reaches 165°F.
Shred the chicken with two forks in the slow cooker.
Cut the tortillas into strips and add them to the chicken and sauce. Stir well.
Mix in 1/2 cup of cheese and half the olives.
Cover and cook on LOW for another 40–60 minutes, until the tortillas soften and the casserole is hot throughout. Top with the remaining cheese and olives before serving.