In small bowl, mix all Yogurt Dressing ingredients with whisk until creamy; set aside.
Cut beef across grain into 4 x 1/4-inch strips. Heat oil in 12-inch nonstick skillet over medium-high heat. Add beef to skillet; sprinkle with Greek seasoning. Cook, about 5 minutes, stirring frequently, until beef is brown. Drain if necessary.
Arrange salad greens on serving platter or individual serving plates. Top with cucumber, onion, tomato and beef. Serve with dressing.