In a small bowl, mix together all of the fajita seasoning ingredients and set aside.
Heat 1 tablespoon of the oil in a large, ovenproof skillet. Toss in the bell peppers and onions and sprinkle on 2 teaspoons of the fajita seasoning. Sauté until the veggies have softened, about 6 minutes.
Using a sharp knife, slice into the side of a chicken breast to form a large pocket, being careful not to cut all the way through.
Spoon in some of the fajita pepper mixture so that the breast is comfortably stuffed.
Season the chicken on both sides with fajita seasoning.
Heat the remaining oil in the skillet and brown the chicken, about 4-5 minutes per side.
Transfer the skillet to the oven and bake for 10-15 minutes, until the meat is cooked through.
Let the chicken rest for 5 minutes before slicing.