The fajita was born when Mexican cowboys, the vaqueros, figured out how to cook a cut of meat called arracheras, or skirt steak, which their bosses had rejected because it was too tough. The cowboys grilled this cast-off meat over a very hot wood fire, bringing out its robust flavor. The citrus marinade, which tenderizes the meat, is adapted from Texas Home Cooking, one of five excellent books on Southwestern cooking by Cheryl and Bill Jamison (The Harvard Common Press).
Picky eaters might just want to wrap the meat in a tortilla and leave it at that, while the more adventuresome can stuff their tortillas with all the fixings.
To prepare the marinade, grate 1 teaspoon of zest from the orange and 2 teaspoons of zest from the lemons and transfer to a small bowl. Next, squeeze the juice from the orange and lemons and add to the zest. Add the remaining ingredients and stir the marinade until well combined; set aside.
To prepare the meat, trim the fat and membranes from the skirt steak or remove the skin from the chicken breast; lay the meat in a shallow baking dish. Pour the marinade over the meat, cover with plastic wrap, and refrigerate for 1 to 2 hours.
While the meat marinates, start your grill. Slice the peppers and onions into 1/4- to 1/2-inch-thick rounds. After the meat has marinated for at least 1 hour (but not more than 2), drain the marinade and cook the skirt steak or chicken on the grill, 3 inches from the hot flame, for about 5 minutes on each side.
Brush the peppers and onions with vegetable oil and place them on the edges of the grill. Cook until tender.
When the meat is cooked to your liking, remove it from the grill and allow it to sit for a few minutes. Then thinly slice the beef diagonally across the grain.
Set out the meat, grilled onions and peppers, tortillas, salsa, guacamole and cheese on your table, and let your family members choose their own fillings and wrap up their tortillas. Makes about 8 to 10 fajitas, enough for 4 people.