Combine lemon juice, oil, 1/2 t of the garlic, mustard and oregano in a gallon zip top plastic bag.
Add chicken and seal bag.
Turn bag and gently move chicken around to distribute marinade evenly.
Refrigerate at least 1 hour.
In a small bowl combine cucumber, yogurt, dill, sour cream, lemon juice and remaining garlic. Cover and refrigerate until serving. (NOTE: Cucumbers have a lot of moisture in them so if you want your sauce to be less runny really drain them well and squeeze the water out if you want.). Drain chicken, discarding marinade.
In a large nonstick skillet saute the chicken for 7-8 minutes or until juices run clear.
Spoon half the chicken on each pita and top with cucumber mixture and sliced onion.