Place the chicken breasts between two sheets of parchment or wax paper. Use a meat mallet to pound the breasts to an even 1/2" thickness.
In a large bowl, add the chili powder, paprika, onion powder, garlic powder, cumin, cayenne pepper and salt and stir to combine. Add the chicken breasts and toss to coat evenly. Set aside.
In a large bowl, add the peppers and onions and toss with 1 Tbsp. vegetable oil. Add the chili powder and salt and toss again until evenly coated.
Add 1/2 Tbsp. of vegetable oil to a 12" cast iron skillet and pre-heat over high heat. When hot, add the onions and peppers and sauté until softened and slightly charred (approx. 4-5 min). Transfer the onions and peppers to a large plate and set aside.
Lower the heat to medium-high and add 1/2 Tbsp. of vegetable oil to the skillet. Sear the chicken for about 8 minutes on each side, until cooked thoroughly(depends on the thickness of your breasts). Chicken should no longer be pink inside and will have a nice char on the outside.
Remove the chicken to a cutting board and let rest for 5 minutes. While resting, place the peppers and onions back into the pan to re-heat for about 1 minute.
Slice the chicken into strips and serve alongside the peppers and onions. Add your desired toppings and enjoy immediately.