Pound beef with a meat mallet to tenderize. In a large bowl, mix beer, lime juice and 2 tablespoons oil until blended. Add beef to marinade; turn to coat. Refrigerate, covered, at least 30 minutes., Meanwhile, cut partially through onions, leaving tops intact. Drizzle with remaining oil; sprinkle with 1/4 teaspoon salt and 1/4 teaspoon pepper., Drain beef, discarding marinade; sprinkle with the remaining salt and pepper. On a greased grill rack, grill steaks and onions, covered, over medium heat or broil 4 in. from heat until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°) and onions are crisp-tender, 2-4 minutes on each side. Cut steak diagonally across the grain into thin slices. Serve with tortillas, onions, cilantro and lime wedges.