1. Preheat oven to 450F.
2. Heat 1 tablespoon oil in a large skillet over medium heat.
3. Add leek and cook, stirring, until beginning to soften, about 3 minutes.
4. Add apple, chopped thyme, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook, stirring occasionally, until the apple is beginning to soften, about 2 minutes.
5. Transfer the mixture to a bowl to cool. Rinse out the pan.To butterfly the tenderloin, lay it on a large cutting board. Holding the knife blade flat and parallel to the board, make a lengthwise cut through the center of the meat, stopping short of the opposite edge so that the tenderloin remains in one piece. Open as you would a book. Cover with plastic wrap. With a meat mallet, rolling pin or heavy pan, pound the pork to an even 1/4-inch thickness.
6. Spread the apple mixture in the center of the pork, leaving a 1-inch border all around. Starting at a long side, roll up the pork to enclose the filling. To keep the stuffing from falling out during roasting, fold in about 1 inch of the two short ends. Tie kitchen string firmly lengthwise around the roast to secure the two ends. Then tie it crosswise with string at 2-inch intervals. Lightly brush the roast with 1 teaspoon oil and sprinkle with the remaining 1/4 teaspoon salt and 1/2 teaspoon pepper.
7. Heat the remaining 1 tablespoon oil in the skillet over medium-high heat. Reduce the heat to medium and brown the roast on all sides, about 4 minutes total.
8. Transfer the roast to a rimmed baking sheet (set the pan aside).
9. Place in the oven and roast until an instant-read thermometer inserted into the thickest part registers 145F, about 15 minutes.
10. Let rest on a clean cutting board for 5 minutes.Meanwhile, prepare the sauce. Crush garlic with the flat side of a knife. Return the pan to medium-high heat.
11. Add applejack (or apple brandy), thyme sprig and the garlic; bring to a boil and cook for 1 minute.
12. Whisk cider and cornstarch and add to the pan. Return to a boil and cook, stirring occasionally, until thickened and reduced by just over half (to about 3/4 cup), 8 to 10 minutes.
13. Remove from the heat; discard the garlic and thyme.
14. Whisk in mustard and any juice from the baking sheet. Slice the pork and serve with the sauce.