Place the chicken breasts in the bottom of the slow cooker insert.
Sprinkle the taco seasoning evenly over the chicken.
Top with the diced onion and then pour the tomatoes over the top.
Finally, top with the undrained green chiles. Don't stir.
Cover and cook on LOW for 6 to 8 hours or on HIGH for 4 to 5 hours.
Remove the lid from the slow cooker and shred the chicken with two forks.
Add the cilantro and mix to combine.
Serve on tortillas or burrito bowls with the toppings of your choice.
