Heat a tablespoon of cooking oil in a skillet with medium heat. Add the diced onion and pepper and stir. Sprinkle with a little taco (about a teaspoon) seasoning and cook, stirring, until vegetables are soft. Add the cream cheese and stir to incorporate. Turn off the heat, transfer to a bowl and stir in ½ cup shredded cheese. Wipe out the skillet to remove any cheese.

With a sharp knife, slice lengthwise into each chicken breast to form a pocket. Place 2-3 tablespoons of the cheese mixture into each pocket. If needed, use a couple toothpicks to help keep the edges of the pocket together during cooking. Sprinkle both sides of each breast with remaining taco seasoning.