We don't make a sauce. Instead, I buy fajita seasoning and rub it onto my chicken (with a little olive oil, so it doesn't stick) before I cook it. Also add it to the veggies before grilling. We squeeze some lime juice on when grilling too. I adore our fajita-grilled chicken!
Then I make the fajitas with a little light sour cream, chicken, veggies, and topped with pico. I also heat the tortillas in a pan.
Yummy... we just made them this weekend and I had a cold version for lunch today.