Cut the chicken into chunky cubes size of 4-5 cm.
Place the olive oil, milk, tomato paste, red pepper paste (if using), chili flakes, paprika, freshly ground black pepper, and salt in a bowl and whisk until combined.
Add the chicken pieces into the bowl and give it a toss until all the chicken pieces are covered with the marinade.
Cover the bowl, place in the fridge, and let the chicken marinate for a minimum of 2 hours or preferably overnight.
Peel the onion and cut into chunky cubes similar size to chicken.
Wash the tomatoes and cut them into wedges.
Cut the Turkish chili peppers into 4 or if using bell peppers, cut them into chunky cubes similar size to chicken.
Thread the chicken and vegetables onto metal or soaked bamboo kebab skewers.
Cook the kebabs on the BBQ for 12-14 minutes or in the 410℉ (210°C) preheated oven grill for 15 minutes, until the core temperature reaches 165°F (74°C).