Warm a skillet over medium heat and add the oil, then cook the chicken with the taco seasoning for 4 minutes. The chicken should start losing its raw look and the seasoning should coat the pieces evenly.

Add the diced bell pepper and diced onion to the skillet and cook for 5 minutes more. The onions should look translucent and lightly sweetened, and the chicken should be fully cooked with a little color.

Transfer the chicken mixture to a plate, then set a clean skillet over medium heat with a small amount of butter or oil. Using a clean pan helps the tortillas brown evenly instead of steaming over leftover moisture.

Lay one tortilla in the skillet, sprinkle Mexican cheese over one half, spoon on some chicken filling, then add more cheese and fold the tortilla closed. Keep the filling in a neat half moon so it stays inside when you flip it.

Cook for a few minutes on the first side, then turn and cook the second side for a few minutes more. The tortilla should be golden brown on both sides, sound lightly crackly in the pan, and the cheese should be fully melted.

Repeat with the remaining tortillas, adding a little more butter as needed, then serve warm. If a little cheese spills onto the pan and crisps up, that crunchy bit is a bonus.