Combine all the ingredients for the peach salsa in a bowl. Season with salt and pepper. Set aside.
For the fajitas: Place a cast iron fajita skillet on the centre rack of the oven. Preheat the oven to 200°C (400°F). This step is optional.
Preheat the grill, setting the burners to high. Oil the grate.
On a large plate, coat the chicken with the oil and season with the chili powder. Lightly oil the bell peppers and onion slices. Season with salt and pepper.
Grill the chicken for 5 minutes on each side or until cooked through when it reaches an internal temperature of 74°C (165°F). At the same time, grill the bell peppers and onion until the vegetables are tender, about 8 to 10 minutes. Remove the fajita skillet from the oven. Be careful when handling the cast iron skillet - it will be HOT. Use a kitchen towel or heat-resistant gloves when handling the cast-iron skillet. Remember, don’t put the hot cast-iron pan directly on your countertop or cutting boards. Use a trivet or set it on a baking tray. Transfer the chicken and vegetables to the hot skillet.
Quickly grill the tortillas. Wrap in a clean dishcloth to keep warm.
On a work surface, thinly slice the chicken and bell peppers. Return to the hot fajita skillet.
Place the hot skillet, tortillas, peach salsa, avocado, cheese, and sour cream at the centre of the table. Let everyone garnish their own fajitas.