Potato Apple Gratin: Grease an 8-cup (20 cm) glass baking dish. In a small saucepan, melt butter over medium heat. Stir in flour; cook, stirring, for 1 minute. Whisk in milk, garlic, rosemary, salt and pepper. Bring to a boil; reduce heat and simmer, whisking constantly, until slightly thickened, about 5 minutes. Whisk in 3 tbsp (45 mL) of the cheese.
Pour one-quarter of the milk mixture in prepared dish to coat bottom. Layer one-third of the potatoes and half of the apples. Drizzle with one-quarter of remaining milk mixture. Repeat layers, ending with potatoes and topping with remaining milk mixture, pressing potatoes down to coat. Sprinkle with remaining cheese.
Cover with foil and bake in a preheated 375°F (190°C) oven for 45 minutes. Remove foil and continue baking until tender and golden, 20 to 30 minutes. Let stand 5 minutes before serving.
Roast Pork Tenderloin: Meanwhile, sprinkle pork all over with salt and pepper. In a large ovenproof skillet, heat oil over medium-high heat; brown pork all over. Transfer skillet to a 375°F (190°C) oven and roast until pork is just pink inside and meat thermometer inserted in centre registers 160°F (71°C), about 20 minutes. Transfer pork to cutting board and tent with foil for 5 minutes before slicing.
Return skillet to stovetop. Add cider and all but 1 tbsp (15 mL) of the broth; bring to a boil over medium-high heat, stirring to scrape up brown bits from bottom of pan. Boil until liquid is reduced by one-third, about 3 minutes.
In a small bowl, combine cornstarch with reserved broth; whisk into sauce and cook, stirring, until thickened, about 30 seconds. Remove from heat.
Slice pork into thick slices; serve with Potato Apple Gratin and cider sauce.
Per serving: 436 calories, 38 g protein, 14 g fat (6 g saturated fat), 41 g carbohydrates, 4 g fibre, 110 mg cholesterol, 427 mg sodium