Preheat your oven to a warm 400°F (200°C). Drizzle the olive oil into a large casserole dish, spreading it evenly.
Add the thinly sliced bell peppers and onions to the dish. Cover it and bake for 15 minutes to gently soften the vegetables.
Carefully remove the dish from the oven. Stir in half of the fajita seasoning with the softened vegetables until well combined.
While the veggies are baking, carefully slice each chicken breast ¾ of the way through horizontally, creating a neat pocket without cutting all the way through.
Evenly divide the seasoned onion and pepper mixture and carefully stuff it into the pockets of each chicken breast.
Place the stuffed chicken breasts back into the casserole dish. Sprinkle the remaining fajita seasoning generously over the tops of the chicken.
Bake for 35-40 minutes, or until the chicken is thoroughly cooked through and reaches an internal temperature of 165°F (74°C).
Optional Cheesy Finish: If desired, for the last 10 minutes of baking, place a slice of light cheddar cheese on top of each chicken breast to melt into gooey perfection. Serve hot and enjoy!