Heat chicken in skillet over medium heat, until heated through. Remove from heat.
Add onions and chilis to pan and cook until softened, about 5 minutes. Add chicken back to skillet and mix until thoroughly combined.
Preheat oven to 350°F. Place chicken mixture in bottom of cast iron skillet, leaving out some for topping at the end.
Cover chicken mixture with cheese mixture. Bake in oven for 10-12 minutes or until cheese is melted and bubbling.
Remove skillet from oven and top with additional chicken mixture, Pico de Gallo, and cilantro.
