1In a blender container, combine sherry cooking wine and remaining ingredients except meat. Cover and run on high until smooth.
2Trim steak of any visible fat. Place steak in a non-metallic baking dish or sealable plastic bag. Add marinade; turn meat to coat. Cover and refrigerate at least 8 hours or overnight, turning meat one or more times while marinating.
3Preheat grill to medium-high heat. Drain meat and discard marinade. Grill skirt steak about 6 minutes over direct heat with cover closed. Turn meat; grill with cover closed 2 minutes more or until cooked as desired. Flank steak will take longer to cook.
4Slice steak into thin strips across the grain. Serve with warm corn tortillas, if desired.