1Cut beef across grain into bite-size strips. Heat oil in 10-inch nonstick skillet over medium-high heat. Cook beef in oil about 3 minutes, stirring occasionally, until brown. Remove beef from skillet.
2Cook bell peppers and onion in skillet about 3 minutes, stirring occasionally, until bell peppers are crisp-tender. Stir in beef.
3Place salad greens on serving platter. Top with beef mixture. Mix dressing and yogurt in small bowl; drizzle over salad.