To prepare souvlaki, combine the first 6 ingredients in a sealable plastic bag; seal and shake to combine.
Add beef to bag; seal and shake to coat.
Marinate beef in refrigerator for 30 minutes, turning once.
Remove beef from bag, discard marinade.
Thread the beef, onion and bell peppers, alternately onto each of 4 (8-inch) skewers.
Heat a grill pan coated with cooking spray over medium-high heat. Add skewers; cook 8 minutes or until beef is done.
Serve the tzatziki sauce with souvlaki, and pita bread