Start by grabbing a small bowl to create your custom spice mix. Combine the cornstarch, chili powder, kosher salt, smoked paprika, onion powder, garlic powder, cumin, and cayenne pepper if you are using it. Stir everything together until it is a uniform color to ensure every bite of chicken is perfectly seasoned.
Place your large skillet over medium-high heat and drizzle in half of the olive oil. Toss in the minced garlic and sliced onions, cooking them for about 2 to 3 minutes until they start to soften and smell fragrant. Add your sliced bell peppers and half of the spice mixture you just made, cooking for another 3 minutes until tender.
Take your chicken breasts and carefully cut a horizontal slit along the side to create a pocket, being careful not to cut all the way through. Rub the remaining seasoning blend all over the outside of the chicken, then fill each pocket generously with the cooked veggie mixture and shredded cheddar cheese. Secure the open edges with toothpicks.
Using the same skillet you used for the veggies, heat the remaining tablespoon of olive oil over medium-high heat. Place the stuffed chicken breasts carefully into the hot pan and sear them for about 4 to 5 minutes on each side. You want to achieve a nice golden-brown crust on the outside before finishing the cooking process.
Once the chicken is seared, pour 1/4 cup of water into the skillet carefully to avoid splattering. Immediately cover the pan with a lid and reduce the heat to low, letting it cook for 6 to 8 minutes. This steam helps cook the chicken through to the center without drying it out, ensuring the cheese melts perfectly.
When the chicken is fully cooked, remove the toothpicks so no one accidentally bites into one. Arrange the chicken on plates and sprinkle with fresh chopped parsley for a burst of color. Serve immediately with lime wedges on the side for squeezing over the top to cut through the richness.