1. Pound each piece of pork, cut side down, with flat side of meat mallet until 1/2 inch thick. Sprinkle both sides with 1/4 teaspoon of the salt and 1/4 teaspoon of the pepper.
2. Heat 3 teaspoons of the oil in 12-inch skillet over high heat. Add pork and brown 5 minutes, turning once; transfer to a plate.
3. Reduce heat to medium. Add remaining 1 teaspoon oil and the onion to skillet; cook 3 minutes, stirring, until lightly browned. Add cider and vinegar; bring to a boil. Boil 3 minutes, stirring to loosen any browned bits. Stir in apples and rosemary; cover and cook 6 minutes or until apples are just tender. Stir in remaining 1/4 teaspoon salt and 1/4 teaspoon pepper, mustard and the pork slices. Cook 3 minutes more or until pork is just cooked through. Makes 4 servings.