Prep the ingredients. Chop up the chicken breasts, fresh cilantro, and onion. Mince the garlic cloves. Preheat your oven and lightly grease a baking dish with extra olive oil.
Cook. In a large skillet over medium heat, saute the onion and cook for 5 minutes. Add the garlic and cook for another 30 seconds. Add the tomato sauce, chili powder, and cumin. Mix to combine.
Combine the ingredients. Remove the sauce from the heat. Use a separate bowl and combine the chicken, cooked sauce, diced green chili peppers, cilantro, and shredded cheddar cheese.
Finish it all off. Transfer the mixture to the prepared baking dish. Add the cheese. Sprinkle with remaining cheddar cheese and then bake for 10 minutes (or until the cheese has melted).