I will often take fat free sour cream, mix with a little chicken broth to get a "sauce-like" consistency, then add salt, chili powder, cumin, cayenne, tabasco, and chipotle chili powder (all to taste) to make a creamy, spicy, smoky topper. It's my favorite fajita sauce, full out, and adds very few calories to the meal.
I use a dry fajita seasoning. You get it where you find gravy mix or taco seasoning. It comes in a packet and if I am just doing enough for me I sprinkles 1/3 of the pack on my meat and vegies as it is cooking and maybe 1/4 cup or water. I let it cook down and stir it to make sure it all gets seasoned. If you are doing it for your family, use a whole pack or more depending on how many people. Then you can just play around with the water. You want enough so it helps the seasoning mix, but not too much because you want it to cook off.
I don't really use a sauce, but marinate the chicken (cut into strips) in a mixture of balsamic vinegar (4Tb), soy sauce (1Tb), garlic (2 cloves chopped), lime juice (2Tb) and chili powder (1Tb). That's for 2 servings. It sounds weird, but it works. Then when I serve them I put the stir-fried chicken/onions/peppers in one bowl and pass homemade salsa and shredded cheese to add to them. I admit that I'm not big on dairy products so when I make it for others I have to remember to be sure to get some sour cream.
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.
We use Lawry's Mesquite Lime Marinade. It is wonderful for chicken or beef or pork. I think it is no-fat but does have quite a bit of sodium. Well, you can't win 'em all!
I bought a dry mix that you add water to, and then added it all to the grilled chicken, onion, and peppers. It was a little watered down, but I liked it. My kids thought it was a little spicy - although in all reality it wasn't. A dollop of ff sour cream and rolled in whole wheat tortillas, and it was a good supper!