Combine yogurt, lemon juice and garlic in medium bowl. Place beef steak and 3/4 cup yogurt mixture in food-safe plastic bag; turn steak to coat. Close bag securely and marinate in refrigerator 6 hours or as long as overnight.
Meanwhile, cut cucumber in half lengthwise; cut one half crosswise into 1/4-inch slices and reserve. Chop remaining cucumber half into 1/4-inch pieces. Add chopped cucumber to remaining yogurt mixture; season with salt, as desired. Cover and refrigerate.
Remove steaks from marinade; discard marinade. Place steak on rack of broiler pan so surface of beef is 2 to 3 inches from heat. Broil 13 to 18 minutes for medium rare (145°F) doneness, turning once. (Do not overcook.) Carve steak into thin slices. Season with salt and pepper, as desired.
Spread pitas with hummus. Evenly top with sliced cucumbers, tomatoes and steak slices. Sprinkle with olives and feta cheese. Serve with reserved yogurt mixture.
Nutrition information per serving: 428 Calories; 90 kcal Calories from fat; 10 g Total Fat; 3 g Saturated Fat; 4 g Monounsaturated Fat; 76 mg Cholesterol; 592 mg Sodium; 42 g Total Carbohydrate; 4 g Dietary Fiber; 40 g Protein; 4.9 mg Iron; 12.3 mg NE Niacin; 0.5 mg Vitamin B6; 1.6 mcg Vitamin B12; 6 mg Zinc; 48.9 mcg Selenium; 114.8 mg Choline;