Prepare chimichurri sauce. In food processor fitted with chopping blade,
process onion and garlic until finely chopped. Add parsley and oil; process
until pureed. Add lime juice, vinegar, salt and red pepper. Process just until
mixed and set aside.
In shallow nonmetal baking dish or sealable large plastic food storage bag,
pour 1/2 cup chimichurri sauce. Set remaining sauce aside.
Place steak in dish or bag; turn to coat with sauce. Cover dish or seal
bag and refrigerate steak to marinate at least 4 hours but no longer than 24
Pour remaining sauce into jar; cover and refrigerate to serve with steak.
Just before serving, heat broiler or grill. Remove steak from marinade and
broil steak 3 to 4 inches from heat source 12 to 15 minutes for medium doneness,
turning once. Discard marinade from dish or bag.
Transfer steak to serving platter; let stand for 5 minutes.
Cut steak crosswise into thin slices. Sprinkle with additional chopped parsley,
if desired, and serve with remaining sauce.