Mix the chili powder, cayenne powder, lime juice and salt in a small bowl. Pour into a zip-top bag large enough to fit the two chicken breasts
Place the raw chicken breasts into the bag, close it and rub the bag to thoroughly incorporate the marinade with the chicken. Place the marinated chicken in the refrigerator for 1 hour.
Cooking the Fajitas
Place a skillet (preferably cast iron) on the stove over medium high heat until hot, about 3 minutes.
Add the oil to the skillet. When the oil is loose and shimmering, add the bell peppers and onions, chili powder, cayenne powder and salt. Cook until the peppers and onions are just brown on the edges, about 5 minutes.
Move the onions and peppers to the outside of the skillet to make room for the chicken. Remove the chicken from the zip-top bag and gently place the breasts flat in the center of the skillet. Cook the chicken breasts using tongs or a spatula to flatten them while cooking.
Cook the chicken breasts for about 5 minutes on each side until the centers reach 150°F/66°C. Once cooked, remove the chicken breasts to a warm plate and cover loosely with foil. Let them rest about 5 minutes. Keep the peppers and onions in the skillet and turn the heat on medium low.
After the chicken has rested, slice the chicken breasts into 1/2" slices. Arrange the sliced chicken in a beautiful fan pattern on top of the peppers and onions. Turn up the heat. Once the skillet has started to get a good audible sizzle, remove it from the stove and place it on a heat-safe trivet.
Serve the fajitas with flour tortillas, guacamole, salsa, cheese, fresh jalapeños, sour cream, radishes, cilantro and lime wedges. You can and should warm the tortillas right before serving the dish.