Place the chicken, onion, bell pepper in a resealable bag. In a small bowl, whisk together the fajita seasoning, oil and water. Pour the marinade into the bag and seal tightly. Allow the chicken to marinade in the refrigerator for 1-12 hours.

Preheat the grill.

Remove the chicken, onion and bell pepper from the marinade. Discard the marinade. Place the chicken, onion and bell pepper on the grill. Grill for 3-4 minutes and then turn over. Cook for an additional 4-5 minutes or until the chicken is cook through. Transfer to a platter, cover with foil and allow the chicken to rest for 5 minutes. While the chicken rests, place the tortillas on the grill and cook for 1 minute on each side. Transfer to a platter and cover with foil until ready to serve.

Remove the foil and slice the chicken into 1/2 inch slices across the grain. Serve the chicken, onions and peppers with the tortillas. Top with cilantro if desired.