In a bowl, combine the yogurt, oil, and water. Add the shawarma spice mix and stir well. Slice the chicken into strips, rinse thoroughly, and add to the marinade. Cover and refrigerate for 1 to 2 hours.

Remove the chicken from the fridge. In a skillet over medium heat, sauté the marinated chicken until cooked through. Add the onion and stir, then add the tomato and bell pepper. Stir and remove from heat.