1. Preheat oven to 425°.
2. Heat a large ovenproof skillet over medium-high heat. Add oil; swirl to coat. Add onion, apple, and garlic; sauté 5 minutes or until tender. Add vinegar and rosemary; cook 1 minute. Place apple mixture in a small bowl. Wipe pan clean.
3. Slice pork lengthwise, cutting to, but not through, other side. Open halves, laying pork flat. Starting from the center, slice each half lengthwise, cutting to, but not through, other side; open so pork is flat. Place plastic wrap over pork; pound to an even thickness using a meat mallet or small heavy skillet. Sprinkle evenly with 3/8 teaspoon salt and pepper. Spread apple mixture on pork. Roll up, jelly-roll fashion.
4. Return pan to medium-high heat. Coat pan with cooking spray. Add pork, seam side down; cook 4 minutes or until browned, carefully turning occasionally. Place pan in oven. Bake at 425° for 15 minutes or until a thermometer inserted in the center registers 145°. Remove pork from pan; let stand 5 minutes before slicing.
5. Return pan to medium-high heat; add stock, cider, mustard, and remaining 1/8 teaspoon salt, stirring with a whisk. Bring to a boil; cook 2 minutes. Serve over pork.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.