Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish and set it aside.
Slice each bell pepper in half lengthwise and remove the ribs and seeds; arrange the pepper halves in the prepared baking dish.
Heat 1/2 tablespoon olive oil in a large skillet over medium heat. Add the finely chopped onion and cook 3–4 minutes until softened.
Add the shredded cooked chicken, cooked white rice, drained black beans, frozen corn, taco seasoning, and chunky salsa to the skillet; stir until evenly combined and heated through.
Fill each pepper half with a heaping amount of the chicken mixture, packing it gently so each pepper is well stuffed. Place the stuffed peppers upright in the baking dish.
Sprinkle the shredded cheddar cheese evenly over the stuffed peppers. Pour 1/2 cup of water into the bottom of the baking dish and cover the dish tightly with aluminum foil.
Bake covered for 25 minutes. Remove the foil and bake an additional 5–10 minutes until the peppers are tender and the cheese is melted and lightly browned.
Remove from the oven and garnish with chopped fresh cilantro, if desired, before serving.