1In medium heatproof bowl, mix pesto, lemon juice, oregano and pepper flakes; set aside.
2In 12-inch nonstick skillet, heat oil over medium heat. Cook onion and bell peppers in oil 3 to 4 minutes, stirring frequently, until crisp-tender. Stir in pesto mixture. Return mixture to same bowl. Cover; set aside.
3Sprinkle both sides of beef pieces with garlic-pepper blend and salt. Heat same skillet over medium-high heat. Add beef; cook 10 to 12 minutes, turning once, until browned and medium-rare doneness.
4Return vegetable-pesto mixture to skillet with beef. Cook 1 to 2 minutes, stirring occasionally, until vegetables are hot and beef is of desired doneness. Serve pepper mixture over beef.