1. Cut beef into small pieces and toss with Greek seasoning.
2. In a large skillet, heat oil over medium-high heat. Add beef, half at a time, and cook, stirring 2 to 3 minutes or until browned.
4. Combine yogurt, garlic, cucumber and salt in a small bowl.
5. Place beef in pitas with tomatoes, cucumbers and feta. Garnish with mint if desired. Serve with yogurt sauce.