Preheat the grill to medium-high heat (400F) and place the cast iron skillet on the grates to preheat.
Trim excess fat from the chicken breasts, slice into strips, and season with 2 tbsp of fajita seasoning.
Remove chorizo from its casing and seed the jalapeno and bell peppers.
Chop peppers and onions into slices, drizzle with 1 tbsp olive oil, and season with remaining fajita seasoning.
Pour 2 tbsp of oil into the skillet, add chorizo, and cook for 2 minutes, breaking it apart while browning.
Add onion and sliced vegetables to the skillet with chorizo, stirring often for even cooking and caramelization.
Grill chicken strips, turning every couple of minutes, until the internal temperature reaches 165 F.
Add chicken to the skillet and toss with the vegetables and chorizo.
Assemble fajitas on flour or corn torillas with your favorite toppings.
