Fajitas come in an array of types -- from shrimp to veggie to chicken -- but beef is the original sizzling tortilla filler. The beef must be tender to make a satisfying rolled fajita, otherwise you'll bite in and chew -- and chew -- or drag most of your filling, rather than a blissful morsel, into your mouth.
The Right Cut
Fajitas are traditionally made from skirt steak, a flap of meat that hangs down in the area of the cow known as the plate -- near the belly. The meat boasts a rich flavor that's quite beefy and buttery. Skirt steak isn't terribly tender, but its coarse texture soaks up tenderizing marinades readily. When you prep the beef for grilling or searing, don't trim off too much fat from the steak. This layer bastes the meat as it grills, to add flavor and juiciness.
A vibrant marinade that highlights the skirt steak's beefy flavor works best. The marinade must contain some sort of acid -- lime juice or vinegar, for example -- to help break down the connective tissue in the beef that can make it chewy. The acid also adds essential flavor, but too much can be overpowering and overdo the tenderizing effects, resulting in squishy, unpleasant fajita meat.
Opt for equal parts oil to acid -- for oil, use vegetable, olive or grapeseed. The oil helps the spices and garlic in the marinade adhere to the beef and promotes even cooking.
Add a splash of soy sauce, instead of salt, to the marinade because it contains enzymes that help make the meat more tender.
The other flavoring elements are your call -- garlic, chilies, black pepper and cumin are common ingredients.
Marinate the meat from 3 to 10 hours -- it's still edible if marinated longer, but the texture begins to turn mushy.
If you have a particularly thick slice of skirt steak, you can pound it with a meat mallet or Jaccard it -- which requires a special device that pokes micro holes in the steak -- to make it thinner and potentially more tender.
Cook skirt steak over a hot grill or under the broiler. This helps form a crisp exterior and a solidly cooked medium interior of at least 135 degrees Fahrenheit. If you serve skirt steak rare, it'll be slimy and chewy, and taste and feel altogether unpleasant.
Carving skirt steak helps tenderize it further. The muscle fibers form in one very clear direction, known as the grain. If you cut against this grain, the meat slices are noticeably more tender. Don't cut with the lines of the steak, or you'll end up with chewy fajitas.