Pound steak to 1/4-in. thickness. In a shallow dish, combine the onion, oil, lime juice, vinegar, garlic, chili powder, salt and cumin; add beef and turn to coat. Cover; refrigerate for 8 hours or overnight.
Drain and discard marinade. Grill steak, uncovered, over medium heat or broil 4 in. from heat for 3-5 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Slice meat across the grain. Serve on rolls. Top with salsa or guacamole if desired.
Nutrition Facts: 4 ounces (calculated without salsa) equals 423 calories, 20 g fat (5 g saturated fat), 54 mg cholesterol, 479 mg sodium, 32 g carbohydrate, 2 g fiber, 28 g protein.