Pork-and-Apple Sauté

1¼ pounds pork tenderloin, trimmed and cut crosswise into 8 pieces
2 tablespoons red wine vinegar
2 Golden Delicious apples, each cut into 8 wedges
1/2 teaspoon salt, divided
1/2 teaspoon pepper, divided
1 teaspoon chopped fresh rosemary or 1/2 teaspoon dried
4 teaspoons olive oil, divided
1 large red onion, halved, sliced
1 tablespoon spicy brown mustard
1 cup apple cider

1. Pound each piece of pork, cut side down, 1/2 inch thick. Sprinkle both sides with 1/4 teaspoon salt and ¼ teaspoon pepper.
2. Heat 3 teaspoons 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 oil and 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, until apples are just tender. Stir in remaining salt and pepper, mustard and the pork slices. Cook 3 minutes more, until pork is just cooked through.