In a bowl, combine chicken, onion, tomatoes, garlic, black pepper, salt, soy sauce, sour cream, cilantro, and half the Parmesan. Mix well.
Wash the bell peppers and cut each in half lengthwise. Remove seeds and white membrane.
Fill each pepper half with the chicken mixture. Press lightly so the filling sits level.
Preheat the air fryer to 180°C (360°F) for 2 minutes.
Place stuffed pepper halves in the air fryer basket in a single layer. Air fry at 180°C (360°F) for 15 minutes.
Check that the chicken is fully cooked through with no pink remaining. Internal temperature should reach 74°C (165°F).
Sprinkle remaining Parmesan and all of the mozzarella evenly over the top.
Return to the air fryer at 180°C (360°F) for 2 more minutes until cheese melts and turns lightly golden.
Remove carefully and serve hot.