1. Preheat grill to medium-high heat.
2. Combine first 5 ingredients in a medium bowl. Add beef to bowl; toss well to coat. Let stand at room temperature 20 minutes.
3. Thread 4 beef cubes, 4 onion pieces, and 4 tomatoes alternately onto each of 4 (12-inch) skewers. Place kebabs on a grill rack coated with cooking spray; grill 5 minutes on each side or until desired degree of doneness.