In a medium skillet, over medium heat, add 2 tbsp of olive oil and sliced onions. Sauté for 4 minutes until they begin to soften. Add in the minced garlic and cook for a minute until fragrant. 

Add the onions and garlic into a crockpot/slow cooker, followed by the chicken breasts, fire roasted tomatoes, chipotle peppers, cumin, oregano, salt and pepper. Using tongs, mix ingredients until all combined. Set crockpot on low for 6-7 hours.

Remove chicken breasts after time is complete and set on a cutting board. Using two forks, shred the chicken. Add the shredded chicken back into the slow cooker and mix with all of the juices. 

You can make tacos with the shredded chicken tinga by grilling the corn tortillas or you can fry them to make tostadas. Then spread refried beans on the tortilla, followed by shredded lettuce, then the chicken tinga, topped with avocado, cotija cheese, and fresh cilantro.