Cut the chicken into chunky cubes, each about 4-5 cm (1 ½" - 2") in size.
In a bowl, combine olive oil, milk, tomato paste, red pepper paste (if using), chili flakes, paprika, freshly ground black pepper, and salt. Whisk until well combined.
Add the chicken pieces to the bowl and toss them in the marinade until evenly coated.
Cover the bowl, place it in the fridge, and let the chicken marinate for at least 2 hours, preferably overnight.
Peel the onion and cut it into chunky cubes similar in size to the chicken. If using large tomatoes, wash and cut them into wedges. Cut Turkish chili peppers into quarters. If using bell peppers, cut them into chunky cubes similar in size to the chicken.
Thread the chicken and vegetables onto kebab skewers. Try to leave a little space between each piece to allow for even cooking.
Set your air fryer to 392°F (200°C), and place the skewers in the air fryer basket in a single layer. Depending on the size of your air fryer, you might need to cook them in batches.
Cook the skewers for 14-16 minutes, turning them halfway through the cooking time to ensure they cook evenly on all sides. Check with a meat thermometer to make sure the core temperature reaches 74 °C (165 °F) before serving.