I have the fajita plate when we eat Mexican. Just pile the meat and trimmings - usually a few onions and bell peppers together with a little salsa and sour cream, guacamole if you want it and eat it as is. Skip the tortillas, beans and rice. Some of the restaurants will trade those out for a small salad. Definitely not carb free meal, but lower than many of the alternatives. Good Luck!
mb