In a large resealable plastic bag, combine the lemon juice, oil, 1/2 teaspoon garlic, mustard and oregano; add chicken. Seal bag and turn to coat; refrigerate for at least 1 hour. In a small bowl, combine the cucumber, yogurt, dill and remaining garlic; cover and refrigerate until serving. , Drain and discard marinade. In a large nonstick skillet, cook and stir the chicken for 7-8 minutes or until no longer pink. Spoon onto pita breads. Top with yogurt mixture and onion; fold in half.