Trim fat from pork. Partially freeze pork. Thinly slice into bite-size strips. Place pork in a plastic bag set into a deep bowl. For marinade, combine oil, lemon juice, mustard, garlic, oregano, and thyme; mix well. Pour over pork in bag. Seal bag. Marinate in the refrigerator 6 to 24 hours, turning bag occasionally. For sauce, in a small bowl combine yogurt, cucumber, dillweed, and seasoned salt. Cover and chill 6 to 24 hours.
At serving time, drain pork. Discard marinade. In a large skillet, stir-fry pork, half at a time, over medium-high heat about 3 minutes or until pork is no longer pink. Using a slotted spoon, remove pork from skillet; spoon into pita halves. Stir cucumber sauce; spoon atop meat. If desired, garnish with onion and tomato. Makes 6 servings.