In a large skillet, brown chicken in oil. Add onion; cook and stir until tender, 6-8 minutes. Add garlic; cook 1 minute longer. Transfer mixture to a 3-qt. slow cooker., Add the next 10 ingredients. Cover and cook on low, 4-5 hours., Remove chicken and shred with 2 forks; return to slow cooker. Stir in cilantro. If desired, serve with toppings.