In a small bowl, combine cornstarch, chili powder, salt, smoked paprika, onion powder, garlic powder, cayenne pepper, and ground cumin. Mix well to create a uniform seasoning blend. This homemade fajita seasoning will infuse the chicken and vegetables with authentic Tex-Mex flavors.
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the minced garlic and sliced onion, sautéing for 2–3 minutes until fragrant and softened. Add the sliced bell peppers and half of the fajita seasoning mix. Cook for an additional 3 minutes, stirring occasionally. Once done, remove from heat and set aside.
Using a sharp knife, cut a lengthwise pocket into each chicken breast, being careful not to slice all the way through. Rub the remaining fajita seasoning evenly over the chicken breasts, both inside and out. Stuff each pocket with the sautéed pepper-onion mixture and a generous amount of shredded cheddar cheese. Secure the openings with toothpicks to hold the filling in place.
In the same skillet, heat the remaining 1 tablespoon of olive oil over medium-high heat. Add the stuffed chicken breasts and sear for 4–5 minutes on each side until golden brown. Add ¼ cup of water to the skillet, cover, and reduce heat to low. Let the chicken cook for an additional 6–8 minutes, or until the internal temperature reaches 165°F (74°C).
Remove the toothpicks from the chicken breasts. Garnish with freshly chopped parsley and serve with lime wedges on the side for an added burst of freshness. Enjoy your flavorful fajita stuffed chicken breasts!